Do you help raise your daughter ?! See her often? Do you financially support your child ?
Her mother ... you said BPD do you mean Bipolar (BP) or Borderline personality disorder (BPD) ?! Does she have a Therapist ? You said she’s on meds , that’s good ... When she leaves does she stay in the area or go far away , does she get in touch with you ? Do you call her?
As for marriage ?? Yes more people divorce these days , it sucks but it’s just the way things are.
If she truly wanted to be married and you are steadfast against it I don’t see how you two could have had a healthy relationship.
Personally when I was younger I had no intention of having a child unless I was married. I fully expected my marriage to last but unfortunately it didn’t, I have remarried.
You mention about being on meds? Did they help you in the past ? If so what scares you about taking them again , you admit your not happy... you need to be pro active to find happiness in your life.
You could meet a woman anytime that has no desire to be married and you have a wonderful relationship.
Life is short , find happiness