Struggling to trust my partner after he lied to me and it’s causing more bad episodes.

My partner lied to me a few times about a person that was in his life, and each time he promised me, swore that he was not lying to me, and eventually I wore him down and he confessed that he had lied to me sleeping with this person who he had a really close relationship with. This has caused me so much pain , so many episodes and I ended up in hospital a couple of times. I still have daily thoughts about this person and wonder if he’s doing something behind my back, but most of all I really struggle with believing him sometimes because he used to look me in the eyes and promise he was not lying (but in my heart I knew the truth) and sometimes I feel like my intuition is telling me once again he’s lying to me. Has anyone been through something similar and can give me some advice please? I am starting dbt soon.