Is this reason to end a relationship?

This is kinda long, but please read to the end ... my boyfriend has a female friend who he used to hang out with on occasions, and sometimes we would all hang out together. I found out some time later that they had actually been in an extensive relationship about 4-5 past and when I asked him about it he lied MULTIPLE times until I brought him evidence and then he admitted to their relationship... he said he lied because he knew I'd over-react and it was nothing like that (but I'm thinking, if it was nothing like that, why did you have to lie about it?)... he said he would stop hanging out with her and it was my understanding that they did up until this past weekend, when I saw her name in his phone... they've still been in contact since that time, I just didn't know about it... I already have really bad trust issues from a previous relationship, and we live about 2 hrs apart, and I've expressed my trust issues to him, so to see that they've still been talking has really upset me. All I can think about is the fact that it must be something going on with them for him to be keeping it from me after he said that he'd stop dealing with her. I haven't brought it up to him yet, but do you guys think I should break up with him or try to work it out? ... we've only been together 5 months.