Oh Bonnie, I'm so sorry you're feeling alone. Where do you live? Stopping work is a huge life change and it can be very hard to make new, non-work friends. If your DD is 15 months, you could try going to some activity groups with her, like Tumble Tots, Monkey Music, etc. I did these sorts of things with DS (now 2.5) and although singing infant songs every week made me want to jump off a cliff sometimes, it was a good way to meet a few people (we moved out of London when I was PG and I didn't know a soul here). Most of them were not that exciting, but I did make 2 good friends that way.
We have a swing park nearby and again, although going there every day made me feel a bit like a sad stalker looking for friends, I kept at it until eventually I had made another couple of friends (they couldn't shake me off . Now whenever I see new people there I always go and say hello as I remember how lonely I felt.
I personally find the whole NCT mums coffee morning arrangements really make me cringe as I never seem to have much in common (other than a baby) with the other mums, but over the last 2 years I have managed to find these 3 or 4 people on the same wavelength as me and it's made the world of difference. After DS was born I was so lonely I got quite depressed, but now I have DD (4 months) I feel much more supported.
If you're feeling a bit trapped in the role of SAHM, os there any way you cld carve out some time alone for yourself? Go to gym and put DD in creche, for example? Or if she goes to nursery, maybe even a part time job?
Hate thinking of you feeling all lonely. Do you ever get the chane to go out with DH for an evening? A lot of my friends have a regular "date night" with their DHs/DPs. Sounds unromantic, but then who manages much spontaneous romance with young kids around?!