I think you have the right point of view: that you are not his doctor, that you can't diagnose him or commit him to therapy. If you want to stay with him I think you have to accept a certain amount of lack. I don't know if you've done any reading about being the partner of a pwBPD but basically you need to be the emotional leader in the relationship. I have also read that you have to accept your partner's limitations and can't expect them to act like a "normal" person. A relationship with a upwBPD (undiagnosed pwBPD) is going to be hard and limited and it will be up to you to set the pace. I've read its really important to have outside interests and stay busy. Its kind of the same advice that is given to partners and spouses of alcoholics.