How to deal with the aftermath of a relationship?

I was with someone with BPD for 6 years and engaged. They went untreated for most of the relationship and ended up verbally abusing me, cheating on me several times and overall would constantly call me terrible things, acuse me of being a bad partner and project their worst fears onto me. It was a cycle I was stuck in. We broke up and weeks later they went back to an ex who they texted during our relationship. This person (said ex) treated them like shit and cheated on them which just makes me think why did they put me through all this just to go back to an awful person? My entire sense of self and worth has been diminished. The pain and betrayal is so hurtful its hard to get up most days. How are you dealing with it all? I start therapy this week but its been so difficult.