Twinmam, this is all so common. HOw you are feeling is totally normal. I could have written your post; every part of it. I love my daughters fiercely and would never want to be without them - I'd chose twins over and over not because it's easypeasy and no bother but because it's the only way I know and the only way to have them both. I'm not an idiot though. I know that one baby at a time would be much, much easier and I know that I would have been a totally different mother had I just had one baby - I'd have been able to get out and about a lot more, I wouldn't have struggled so much.
But it does get better. The first year is really hard as they aren't mobile, you have to do EVERYTHING physical twice and deal with lugging two babies around. So normal things that mothers of singletons take for granted (taking them out in the pram, taking them to the car in the rain, nipping into a coffee shop or cafe for lunch/coffee, etc. are all SO much harder with two babies. I found it incredibly isolating and I struggled with the guilt. They got half of a frazzled mother all the time and we didn't do as much as other babies. We went out and I dragged myself to playgroups and baby classes but it was a struggle and we certainly didn't get out as much as friends with one baby did.
All you have to remind yourself is that THIS is the tough part. The first year is really tough and everyone finds it this way - you are no different or worse than any other twin mother. In fact, you're totally normal. You also need to remember that your babies know no other life and no other mother. They've never known what it is like to be a single baby and have 100% of your attention so they don't miss it. As far as they are concerned, you are their mother, you are the BIGGEST love of their life and that's all they need. Someone once told me that 'no baby is born patient but twins have to learn it and that's no bad thing' which I found really comforting. My daughters got less of me than they might otherwise have done and they sometimes had to wait for a feed or a cuddle or a nappy change but they're patient for it and very chilled as a result. It has done them no harm and in fact I think has made them much better at sharing and patience, which is great in the long run.
Honestly, you sound a bit raw and I totally empathise as I found the first year a real struggle, especially as my daughters got bigger and heavier. THey were late to sit up, move and walk so I really bore the brunt of that. But you just grit your teeth and remember that after a year, when they start to play together you'll begin to reap the rewards and mothers of singletons will start to envy you; because you'll have children with instant playmates who'll be happy to play together rather than demanding all your time (my friends are green with envy at me about this). Plus, as it gets easier the harder days will get fewer and you'll be able to relax into it a bit more.
I'd chat to your GP. I can't say if you've got PND but it might be worth having a chat with him/her as there is no timetable for this - it's not always something that pops up straight after the birth (8 months post can be quite a common time) and there is help available. Also, on the days it is all hopeless just treat it as a day to get through. I always found no two days were the same and if I had a really crap day the next one was always better. So treating each day as individual rather than 'oh my god I've got months of this...' can help. As can forcing yourself to go out. It sounds stupid but I found the more I got out the better I felt about myself. It was sometimes a huge struggle to get out of the front door but even walking to the supermarket made me feel a bit better. I used to have to force myself to do it sometimes and, although it sounds stupid, I used to imagine that I was an 'ambassador' for other twin mothers. THat by getting out of the front door I might be seen by another struggling twin Mum who would take heart from someone else getting out there and getting through a difficult day. It sounds a bit mad, but it did help - thinking that I had to do it for other people.
Finally, and this has been an epic of waffle, you are allowed to have rough days and to struggle. It is really, really hard and the rest of the world doesn't get what it's like. Until you're there, struggling and juggling twin babies not even the best and sympathetic imagination can grasp the slog that can be involved. But you belong to the BEST club in the world. Twins are the stuff of magic and you WILL reap the rewards of that. You're doing it and you're doing really, really well so just remind yourself of that. You've got through the first nine months of twins which makes you, officially, superhuman and supermummy.
Hang in there and remember, we've all been there and you will come out the other side. It does get easier.