My situation was kind of the same for a very long time. No real long term serious relationships, the longest being about a year, all ending badly.

No cause for my bouts of depression, other than genetics.

I just got married for the first time and I'm 44. I spend many long years living alone and wanting to be in a relationship. It sucks.

Lots and lots of self-loathing.

Therapy and meds helped a lot. But it didn't help with the relationship and loneliness problems.

The fact is depression just makes it harder to form and maintain relationships. It's not your fault.