It can be very hard, and I think all of us who are single mums have been in the pits of despair (I tend to get to rock bottom when I'm ill). It is the hardest job in the world by a long way - I go out to my job to get a rest. Avenue has sound advice - totally agree with the "get out every day" suggestions and "invite a friend round once a week". I also have phases where I go to bed when DS does and it really helps.
Adult company for you is crucial- you will go loopy without it (during my maternity leave, I spent lots of time talking to little old ladies on park benches). I find friends don't come to me uninvited unless I put in a lot of work in terms of inviting them round first - I have a hit rate of about 2 events organised by me for every event I get invited to, but it's worth it for my sanity. And other mums, when you get to know them, are usually equally pleased to have some adult company, and will be happy with hospitality consisting of beans on toast for them and their DC (not kidding about this one - try to find a bunch of friends who're all happy to set the catering bar comfortingly low).
The other thing is organisation - work out when you have most energy and fit tasks round it. For instance when DS was a colicky baby, I knew that come 4.30-5.00 ish, things were screwed for the rest of the night, so I had a cooked meal at lunch time and stuff prepared to snack on in the evening between screaming fits. Toddlers can be similar - no colic, thank god, but if you know for instance that he's going to have a high-demand patch in the run up to his tea, make sure tea is something that can just be got out of the fridge and plonked in front of him, rather than taking loads of preparation.
Also try to find stuff you know you actually find ok - I find going to the park is really good - DS runs off energy, I get some fresh air and daylight, it's an activity where we both know what we're supposed to do, if you know what I mean (contrast playing with toy cars which I hate with a passion). For wet days, there's libraries, our local museum (don't know what your town has to offer, but the trick is to find somewhere free, warm and with child-friendly activities).
Also (fingers crossed the evil tories don't knacker this one) when he turns 3 you will get 15 hours free childcare, which will really help a lot.