I very rarely say anything on here, but your post made me think back to feeling exactly like that. I divorced nine years ago, and for ages I felt very much as you describe. I was mostly a SAHM, with a small business with a friend, but mainly just for interest and a bit of pin money.
When my husband left me, I felt like I didn't really know who I was any more, and I realised (after a long time) that I had come to define myself almost entirely in relation to other people - someone's wife, mother, daughter etc. It wasn't until I started to do things for myself that I started to feel more positive and gave myself new purpose in life.
I did have a couple of relationships, but my confidence and self esteem had taken a battering over the years, and they were ultimately doomed.
Getting out and working made a huge difference to me - only part time, but enough to see myself again as a person in my own right, outside the home, doing something I had created and earning not just financially, but self respect again. It wasn't about the money, it was about me. I did feel a bit selfish at first, and worried that I might be disadvantaging my children by not being at home with them as much, but they have benefitted from seeing their mum as the strong independent woman I always was inside, but had somehow lost sight of.
And my restored self esteem helped my relationships too. I met a man (online - as you point out, friends, though great, are busy and social life suffers) who is just lovely, treats me with respect and kindness, and has recently moved in. After 9 years living as a single mum, I never thought it would happen.
But anything is possible - just give yourself permission to discover, and then do, what makes you feel good about yourself.
Sorry if I've gone on, but you touched a nerve!