Boyfriend trouble: help?

About a week ago I found out that my boyfriend was doing stuff with his ex girlfriend) (they grinded at the club, she kissed him a couple of months ago and they were flirting a lot over text (they were doing some bet or game thing and she said if she wins...he has to tell his best friend and me that she and my bf were getting back together). It completely shattered me emotionally and although we had a huge discussion about it and he showed his remorse and i forgave him, i am finding it very hard to trust him. i've been feeling sick over this issue for the past few days now. what can I do?