Hi RufusTFirefly

The problem is, since beginning the relationship, I've become more and more isolated because A seems to have a problem with all of my friends. And she says it's my fault (everything is always my fault).
This is familiar to many of us here.
1) Can someone reassure me that it's okay to have friends and spend time with them and not feel like I need her approval before I do so?
I assure you! Its not only normal but important to spend time with your friends. Its important for you, its only fair to your friends. Food for thought: Ironically if you end your friendships for the sake of the relationship with A it can feed the mistrust as there is now proof that you can and will discard people.
You dont need her official approval or permission to continue your friendships. It does help to validate her feelings without giving into the request. Heres more on validation:
https://bpdfamily.com/content/communication-skills-dont-be-invalidatingYou can acknowledge her feelings without giving into them. This is important and very helpful to the communication between you.
2) Are there any suggestions for establishing boundaries so that I can start rebuilding old friendships? I'm getting better at defusing fights, but I'm still not particularly good at establishing boundaries with her.
I understand that question to mean that you are trying to get things to a better state between you and A before resuming your friendships. Did I get that right?
If so I wouldnt wait. This is a matter of you staying consistent and true to yourself. Thats the boundary. There is no need to explain anything. Just keep being the friend that you were before this relationship started.
Heres a good article to read about basing our boundaries on our values:
https://bpdfamily.com/content/setting-boundariesHeres a good link about how explaining things can work against us:
https://bpdfamily.com/message_board/index.php?topic=139972.03) One of the ways I was able to diffuse this fight was by telling A that I wouldn't see B at least until A came back from her trip. How should I explain to B that I can't see her without placing all the blame on A?
Well I agree thats a tricky one. Because no matter what she says its still you that makes the decision so she isnt really the one to blame. Do you see how that works? I dont mean that harshly, I completely understand your dilemma. I honestly think most people would do the same thing (including myself) but its really just postponing the problem instead of solving it.
I dont think there is a right or wrong here I can only share with you what I would do. I would call my friend, explain that my SO was having a hard time with the friendship. I would tell my friend I made this decision to buy myself some peace momentarily. Then I would ask my friend to be patient for a limited amount of time. I think if I was honest about everything there is more chance of making things right.
She just considers it cheating for me to be spending any time with B whatsoever.
When your girlfriend gets back from her trip I think it would be really helpful for you to have a discussion about what each one of you considers cheating and what friendships mean to you. Hopefully from that conversation you can clarify your boundaries in regards to your friends but at the same time find a mutual understanding of what fidelity means for you as a couple.
Hope that helps. Keep us posted. We are happy to have you with us.