Forever alone?

So I'm posting this because I feel like I'm at a loss. Since I turned 20 I've felt like I've hit a bump in the road. I suffer with a lot of mental illness's and can't seem to hold any steady relationships or a job past 8 months. Lately (last few months) I haven't even been able to have sex. I was seeing a guy for almost two years it was nothing but sex. I loved him but he never lead me to believe he was interested in me past the bedroom. My last relationship I was in was with my ex gf, who left me for a man not even a week before I was going to propose to her. I've been ready for a couple months now to get back out there, but nothing I have tried has worked. I feel unattractive and lonely as hell. I feel like I'll never get a life started or have kids or anything like that. Any advice?