I think guilt is the mother's disease - no matter what we do we feel guilt over it right from the word go: whether we co-sleep or not, breastfeed or not, use dummies or let them suck thumbs, what age we go back to work, etc. We can not win - someone somewhere will think we're doing the wrong thing or at least we imagine they will!
Whichever size family you have will have its pros and cons as custardo says. I am an only child and loved being an only child while growing up, although as an adult I would have liked a sibling to turn to when my father died. I think the only thing you can do is rejoice at the pros and compensate for the cons as best you can. (I know that's easier said than done, and is cold comfort when you must be feeling bitterly disappointed right now).
I think the dog sounds a great idea - I'm asthmatic so I had a rabbit when I was a kid (although I was a bit older when we got him). A dog would be much more companionly though - make sure you get a breed that is very good with kids (she says, stating the obvious).
Other options... my son is only 18mths and since about 9mths he's been going to a day nursery for one day a week. I have the excuse that I work from home and that one day helps me gets loads more done that I don't do in the evenings, but if you can afford it that might be an option. It gives you a break, and ds really enjoys the activities (there's no way I would do some of the more messy play at home!) and the other children.
Another thing to try is to contact your local Children's Service and NCT groups - they might be able to tell you about other toddler activities in your area which you could take him along to.
Remember in all of this that you need some looking after too. Do try and give yourself a bit of love and time out - it does sound like you've had a lot to deal with the last few years.