Advice needed..

My partner & I have been together for 3 years, we have been through so much in this time. Last year he lied to me about meeting up with one of the girls from his work ( which he has a crush on) he was texting her for a few days asking to catch up and the day they met he lied to me and said he was somewhere else. I found out only because he got a message that evening from her an I happened to be near his phone. I just can't get over it and every time we are apart or his on a day off I keep thinking he will do the same thing, can anyone give me any advice on how to get over this fear?? I get bad anxiety about it. I am trying my best to build trust back.