We've all lost friends along the way and I, like you, have got a bit cynical about letting people close. I've lost friends because I split up with my husband (not real friends), I lost friends because I met someone else and it didn't fit in with them (not real friends).
Evening classes are a great start I think and, I suppose, I have a young child so can get myself involved in a lot of stuff with school governors and coffee mornings.
Work is a good outlet - I work from home and for my father so he is the only person I see. I go on a lot of workshops for school with the governors, do classes in silly stuff as well as stuff that will help me with work, etc.