<human>: I am dating a wonderful lady. We've been dating about 8 months and we're both very much in love. Everything is great except for one issue. 

First here is some back

I broke up with my ex [f25] a year ago. Things didnt end the greatest and we no longer talk, see each other etc... In fact, I have her blocked on FB. I have no desire to talk to her. Before we broke up we, we talked about doing an Ironman triathlon at some point. After we broke up, I kept wanting to do it and still trained for it. Well, apparently my ex did to. She called me very briefly last december and chatted for 20 minutes or so and it turns out we both signed up for the same one. 

Flash forward to January, and I've met this wonderful new girl and we started dating. My current GF has mentioned that she has a problem with me going to do this out of state triathlon because my ex will be there. I have really enjoyed training and really want to go, and I absolutely do not want to see or talk to my ex. I can see where she is coming from, but at the same time, I signed up for it before I even met her and it was very expensive ($600). I've offered to have her come too, but that doesn't seem like it is an option. My GF tells me that if I go she feels like I am choosing me ex or going to this triathlon over her. I understand her feelings, but no amount of communication has lead to any kind of resolve. 

Part of me just wants to throw my hands into the air and just say fine, and not go. It would be the easiest route because I have a very busy grad school semester. But then I might resent her in the future for giving me, what I feel like, is an ultimatum. So then I feel like I almost have to go out of principle. What should I do?

Can you summarize the given text in a few sentences?
<bot>: I signed up for a very expensive out-of-state triathlon that my ex will be at (unknowingly). It bothers my current GF that I want to go. I feel like, she doesn't trust me.