I’ll echo the “see a therapist if it’s feasible” recommendations. Here’s another possibility:

We all have a worldview, and that includes our own understanding of who we are. One of the possible reasons for self-sabotage is when we believe ourselves to be a certain way (or that we do/don’t deserve something), we tend to self correct for any anomalies, whether good or bad. That is, if I was treated poorly as a child, or otherwise lacked the love and support that children need, I might believe myself to be unlovable and “correct” anything that contradicts that understanding (such as someone showing interest in me).

It could be that, or any number of other things. I’ll echo another recommendation: take note of your emotions when you’re in those situations (you might even be able to feel them when you imagine a situation like you described). What do you feel?