Letting Go of Jealousy

Ha
So me and my BF of 2 1/2 years broke up last year at about march. We have recently gotten back together and I am trying to come to terms currently with the fact he slept with someone else in that time frame. 
I am so guilty for feeling so upset, sick and hurt--it was totally fine because we weren't together at the time. I just can't stop thinking about it and looking at the girl's accounts on social media. I am so upset that every time I try to eat I get nauseated. I'm even having trouble getting to where I need to go when masturbating because the thought of them having sex pops up in my mind and I almost vomit.
Please--can anyone offer advice about getting over these feelings?