I have to second Mono on this one. The language you use: relatively, struggled, etc are all qualifying terms for your relationship. Having had a BPD ex I can tell you that it's not BPD that's giving you these feelings, if it was you would be pushing him away more than singing his praises if you are not fulfilled emotionally.
I have said it many times to people talking about being Poly, and I've told all of them that for TP to be monogamous would be restricting a large part of her personality and it would be emotionally unsatisfying for her. She and I had build up trust before I was ready to open the relationship up, but if you've given it a go and it's not working, it's not working.
Polyamory is wrong! It's Multiamory or Polyphilia. Mixing Greek and Latin roots? That's wrong.