Do you have a therapist you can work these feelings out with? As you know I can't diagnose, but I would imagine the feelings you have are a borderline trait more than bipolar. Of course we all know that disorders can overlap.

At any rate it sounds like you want to be happy for your friend, but are fighting with the feelings that you have ingrained in you which are to do anything to stop their relationship. It is really something that can be worked on in therapy or maybe in a dbt program. You could look up DBT and Marsha Linnehan who is the person who came up with it. I have been in a dbt group and I have found the skills helpful.I don't have a borderline diagnosis, but studies are showing that dbt is good for bipolar as well.