(Warning - novella length post ahead)
@Thatagain Unfortunately if you live in pretty much any private rented accommodation (and a lot of public), having a pet that's any bigger than a goldfish isn't usually an option. There are other options, though; our local RSPCA shelter is pretty much always looking for people to help walk the dogs they have in there until they find them homes. (I found out as a colleague in a previous job was doing just this. I couldn't walk the dogs at the time but I was able to volunteer for "cat cuddling" as they called it, I think it's now "spending time with the cats".)
We do now have our own furbabies, I mean cats, and despite having sworn I would never be the kind of mad cat lady who fed her felines tuna or ham, they do get rather spoiled. In fact, we think they possibly eat better than we do...They're nice-natured cats, and not spoiled in any other way, but they do like a good meal and having someone there most of the time.
@User12879923378 You had me crying with laughter at your comment about the early days of your running experiences, when elderly folk and their even more elderly dogs would pass you. It reminded me of a trip to Glastonbury Tor. This lady who must have been in her 70s at least went striding up the hill and by the time I had managed to wheeze up the first third, stopping every time I came to a bench or flat outcrop, she was already lurching down the other side of it (I could see her as a speck in the distance...)
@CherryPavlova Your MIL sounds ace, and a lot like my aunty (sadly no longer with us) who at about the same age was STILL the first one up on the dance floor at pretty much ANY family gathering. My paternal granda lost his legs to gangrene and was in a wheelchair in a residential home all the time I knew him; he still very much had his wits about him, and every time we visited he'd tell us that he was going to marry Kitty, who was one of the other lovely residents. Sadly they never did (my gran had died the year after I was born so he'd been a widower for at least a decade by this point), but his attitude to life has stayed with me and sometimes I have to remind myself of that.
@Inacompletemess, There are a couple of other things I was going to suggest as they really helped me, and were literally only a few pounds a week or free. One was yoga, or Pilates. Not all yoga classes are Iyengar "how long can you stay in one position are you experiencing cramp yet" or Bikram "how hot, sweaty and uncomfortable can I make you". There are lots of other styles; one of my favourites was Vini (though I only ever found one teacher who specialised in it) as it mainly involved a bit of gentle stretching, loads of breathing, and a lot of lying on the exercise mat doing both of the previous. Plus a lovely meditation at the end. I wasn't the only one that often dozed off. And the funny thing was, even though it was so incredibly gentle, I lost weight. At the time I was in a very high-stress job with masses of travelling, and I was eating REALLY badly so my weight was towards the very high end of a size 18. And I'm only just over 5 foot. So not the best, really!
While the breathing is different from yoga, I also found Pilates an excellent form of exercise - less of the "woo" meditation stuff if you're not a fan of that, but still lots of very targeted stretching. The class I went to was run by one of the physiotherapists at the hospital where I then worked, and she was absolutely super. She used to say she taught classes for people with joints, not prima ballerinas. There was a waiting list for her classes, she was so good.
At my lowest, I used to get in from work, lie on the sofa, and put on my Paul McKenna "You Can Change Your Life" CD. (I picked it up for a couple of pounds in a charity shop.) There were a couple of options, I usually used to go for the 20 minute one. It's a lot of visualisation about how you want your life to look in the future, and while I don't think for one minute that it's possible to instantly visualise and create large houses or a life in the Bahamas like they promise in books like The Secret and all, this recording and others definitely helped me change my mindset.
I would also second what others have said about popping in to see your GP and explaining that you're really down at the moment; like many of my friends and family, including some that really surprised me, I've been put on medication for depression and anxiety at times, and while I was very worried at first, it really did help. Your GP might not suggest medication; they might suggest something else for you like a support group or something, but depression is so common these days. (Not surprising when you think how mad our lives are, for most of us!)
Also, getting out into nature; I do enjoy walking (though some of the fittest ramblers I've ever met have been over 60 - I'd say I'm more an ambler than a rambler), and one of my hobbies is photography. I've met great people at camera clubs. You do tend to get more men there than women, overall, and while I didn't meet any romantic partners there, I still had a great time (and a friend of mine who also loves photography met her long-term partner that way when she was in her very late 30s, about your age - they're still together, over a decade later). And I met two previous boyfriends through creative writing courses and classes.
So, I guess, when it comes to the volunteer stuff and/or hobbies, what do you enjoy doing? Or, conversely, do it as an elimination exercise - what do you NOT enjoy doing? (I'd never do anything with team sports, for instance, not because I'm not a team player but because I'm slightly dyspraxic, blind as a bat, and for me, there are better ways to spend a freezing cold Saturday morning than kicking or whacking a ball about a field.)
And you've said you're looking into a second job - that's a brilliant idea. I can understand why bar work or hospitality might not appeal, but what about retail? Even just temporarily, for a few weeks leading up to Christmas. Our local "Range" has had signs up for the last couple of weeks asking for shop assistants, for instance, and that's in a town where we've lost our Marks and Spencers and are about to lose our Argos, but they're still doing okay, as are Dunelm, just next door, and Pets At Home, just opposite. And I think I remember seeing the Post Office advertising for temporary sorters as well. Or a weekend reception job in an estate agent's office?
One of the other things that brings you sadness is the idea of not having a family. Someone I know who adopted (not a particularly close friend) found that having a family doesn't necessarily solve problems, it can just bring more of them. In the form of a small person or two. This particular person hadn't had much experience of being around children and some aspects came as a bit of a shock, shall we say. They'd had something of a rose-tinted view of family life (no siblings).
If your sadness about children is partly the fact that you feel you'd like to contribute to the future of society, and pass something on to the next generation, then are there any homework clubs, or reading schemes, or guiding groups you can volunteer with? Again, I know in some areas guiding groups and the like are crying out for responsible adults to help organise activities and the like. You'd probably need a DBS check (is that the right terminology these days? I struggle to keep up sometimes!) but any such groups could advise you.
We are all sending you positive thoughts in our different ways - some of us taking the chivvying approach, and others the tough love, and others what I'd refer to as the Games Teacher "Oh, for heaven's sake, cannycat20, buck up! Life's not a rehearsal, you know!" approach.
For what it's worth, I don't have my own kids either and I am now genuinely too old to have my own. I adore my nieces and nephews, but I live hundreds of miles away from them so only get to see them a couple of times a year (hoping to move nearer before too long), and one of my sadnesses is that I've missed seeing most of their childhood. So to them I'm fairly sure I'm just this wacky aunt who turns up once or twice a year and then disappears again into the mist....!
On the food issue, others have given excellent advice, and crash diets, as they say, won't work. Changing the way you think about food will. I know it might sound a bit "woo" but whatever you're eating, think about where it came from, and the creatures involved in bringing it to fruition, human and otherwise, and give thanks. When I'm in one of my "away with the fairies" moods and eating, say, apples, I like to think of how they started as seeds, and how they blossom in the spring, and how the bees come along and stick their tongues in (did you know bees have two stomachs, by the way? One for eating and one for storing honey - I only found that out this week). And then the flowers die, and small fruits appear, and then they get picked, and sorted, and packaged, and brought to wherever I buy them from. It's a great distraction technique for when I'm cooking dinner (not my favourite activity).
On the money thing, you can register, free of charge for your Experian account, which will help you keep track of your credit score. You might find it's not as bad as you think - due to illness my income's been a bit erratic the last couple of years - settling now, hopefully - and I was pleasantly surprised, as although I've had a lot of credit, I've paid it back. The Experian account offers you an enhanced first month, and then, as long as you remember to cancel any bank direct debits you've set up, it's free to log in once a month with the account that doesn't have all the bells and whistles.
I also do the Qmee and the Swagbucks and the other things people have mentioned (and new ones are always coming online) - you're not going to get rich quick, but they do have the advantage that you can do them on your phone if you've a few minutes spare if you download the apps, and actually they do mount up. I'll be using what I've earned this year from those towards quite a good chunk of Christmas, for instance.
Don't be so hard on yourself, and do just take things gently, one step at a time. 
