Just relax and take a deep breath. It´s possible you´re making problems bigger by freaking out.
Look at it this way instead. Your partner did something he should not have done, yes. But he has taken active steps to heal his own issues (which seems to be that he has difficulty trusting people) by going to therapy and taking medication. That is a very good step!

Checking ones online activity is not necsessarily bad, we are all different. You clearly think it is wrong and an invasion of your privacy, and he should respect that.