<human>: Basically my gf and I have been officially dating for about 4 months, and the 4 months before that were sort of an extended hooking up, semi-serious relationship.  During that time she went and visited a guy who she told me was a guy she used to hook up with, nothing serious, and she just saw him to officially end things.

I just found out that she was actually in a long distance relationship with that guy, and after that visit they continued to talk daily until our relationship got more serious, at which point she ended things with him (not telling him specifically that it was to date me).  She never came clean about the reality of her relationship with this guy, even though we have talked about it since... so even when we have been seriously dating, she has been dishonest about that relationship and stuck with her story from before, until I came across messages between the two that confirmed it was much more serious and that they had continued to talk all the time as a long distance couple as our relationship was developing...

Basically, i don't know what to do.  She acknowledges that she was dishonest and says she will change and not do that anymore, but it's hard for me to trust her.  I'm honestly not upset that she was with that guy, and I know we werent that serious then, but I am more concerned with her deliberate lies of omission and misrepresentation of the truth.. Also, the fact that she was never going to tell me, and apologizes profusely now only that I found out on my own.  I have also caught her in a few smaller lies, nothing that important, just random things that don't matter about how she spent her time etc that I dont know why she would lie about (she did say that 'lying is fun'.. see edit below though, it wasnt that serious)

Should this be a deal breaker?  Our relationship is really great otherwise, and I was so happy before I found this out.  She says she will do everything she can to earn my trust back and swears she wont be dishonest anymore.

Can you summarize the given text in a few sentences?
<bot>: girlfriend lied about having a boyfriend while we were first together and not too serious.  i just found out and am nervous about her dishonesty