I'm not quite sure where to start. 

I got diagnosed with BPD this year, I'm 23F. I've been in a relationship for the past 4 years and I'm very happy. There are moments where I split on them and I push them away to the point of zero contact and I feel uncomfortable with their affection. This is usually brought on by feelings of jealousy, perceived threats, or other external factors. The other day I had a classmate ask me out on a date through text and I didn't know how to say no. I had my partner help me construct a response that was mature but I still felt bad letting down my classmate. I often neglect to talk about being in a relationship, and in combination with my "flirtatious" personality this leads people to the wrong impression. I think that deep down I want to seem available, but I don't actively seek out other people. It all comes down to validation. Any potential partner (most often cis men) I want to seem available because I love the attention. And I guess since I rejected my classmate I feel at a loss because I lost that sense of validation from them. I have awful anxiety and guilt from this and I can feel myself starting to fixate on my classmate (thinking about them, our interactions, wondering what I would do if I was single, etc.). I'm thinking of ways I can still get that constant validation from them, such as lying about the length or seriousness of my relationship. This current hyper-fixation on my classmate has caused me to slightly split on my partner because I am no longer seeking his validation, I instead want it from this random classmate. I'm wondering if I should wait it out until this feeling goes away, or try and get to the bottom of this issue. 

I know this is unhealthy. I feel safe in this community to share my insecurities and anxiety regarding my current situation. Any advice or shared experience is welcome.