I was diagnosed with bipolar two years ago after suffering from a traumatic event. After the event, I was completely paranoid of everyone around me, had suicidal thoughts, and was extremely depressed and anxious.
I was put on a mood stabilizer that worked great, and the symptoms went away.

Eight months ago I met my girlfriend. She is the most kind and understanding person that I have ever met and it was great finally being in a healthy relationship for once (for the both of us). We have had amazing memories together, have discussed the future (marriage, kids), and honestly believe we are soulmates.

Around a month ago all of my symptoms came back. My mood swings are severe and it has definitely taken its toll on my girlfriend. Whenever we used to see each other she would be so excited to see me, but now she isn’t excited and the initial hello is very plain, like seeing a friend, which kills me inside. Today we sat down and talked because I unknowingly snapped at her over the phone and her friend overheard so she told her she didn’t deserve the way that I treat her, which I completely agree with and understand. She said that she didn’t know how much longer she could put up with it and bawled her eyes out which killed me. Due to whatever is going on with my illness I couldn’t express any emotions or cry even though I was upset. I told her that it might be best for her if we broke up, and she broke down again crying saying that it wasn’t what she wants for us. I feel as if our relationship is strained and as much as I love her with my whole being, I think she would be better off without me. At the same time, she says that she just wants me to be happy and it kills her to see me this way. I can tell that it scares her since I’m usually outgoing and happy but I’m now empty and cold.
 
Though I haven’t told her, I have been extremely paranoid that she is looking for another person to leave me for and it eats away at me. She told me that she wished an old friend a happy birthday, and I see that he has been popping up on her snapchat and she doesn’t open the snaps in front of me, which could mean nothing but of course my gut/paranoia tells me otherwise. It also makes sense in my head because lately I have been clingy, insecure, and she has to reassure me often that everything is okay, which is definitely unattractive traits for me to have right now. We both know that this isn’t how I usually am but in the meantime it sucks terribly.

I am going to therapy and am switching my medication to lithium next week. Any advice on what I should do in the meantime?

Tl;dr - my bipolar is destroying my relationship.