<human>: I found a journal he kept back then and I know I shouldn't have, but I read it. At the time, we had been doing the long distance thing for a couple years and it was taking a toll on me and we split for about 3 months. 

We stayed in contact during that time and he always maintained that he wasn't even thinking about striking up something with someone else. 

Flash forward to this week. I find the journal and learn that not only was he dishonest with me about not dating, but he was really into this girl and would have pursued something long term had she been interested. 

I am devastated and feel very betrayed. Our wedding is next month. A lot of money has been spent that can't be recouped and I have put so much time and effort into planning that it makes me heartsick to think of calling it off until we work through this. 

Am I making a mistake by going forward with it?

Can you give me a shortened version of the text?
<bot>: My fiance lied to me about something 4 years ago that I just found out about and now I'm not sure I should go through with the wedding next month.