I had an affair with this girl for almost 3 years. We dated for 6 months but had to breakup cause there's things in her life that makes it impossible for her to have a relationship. I still love her, and she says she does to. 
I live in a nearby city and not always go back to my hometown (where she lives), but when I do we still hangout and really enjoy our time together. 
We were talking about getting back together and she told me she was working on it and that she would be able to be with me again soon, and it made me very happy. 
Last week I went back to my hometown to go to my friend's bday party, my plans were to smoke, drink, and have a good time with my sister and my friends. When I told her that I was going to a party she started asking me to take her with me. I tried to tell her that she would not appreciate the occasion, and that I'd like to have some time to myself, and that we could grab a beer on the next day and spend the whole day together. 
Of course she refused to accept. She said I was basically telling her that she's not good enough to hang out with me and my friends. I gave up on trying to explain to her that I just wanted to have some time to myself. 
A couple hours before the party I noticed that a few names were added to the invite list. Of course her name was there. I asked I she would really go to the party and she said that she was going with her friends (even though she only knew me and one other person in the entire party). 
When I got there and she saw me she started trying to kiss me but I didn't let her (multiple times). A few minutes after that she came to me holding hand with a guy and asked me if it was ok if she kissed him. 
I told her "I am not your dad. You should do whatever you think is right". 
So she kissed the guy right beside me, while I was watching. 
Needles to say that it ruined my night. After that I think she saw I was mad with her, so she kept trying to talk to me and kiss me for the next our or so, and then she left the party.
 Now she's telling me that she was not trying to hurt me, that she thought that I would not care, and that she was also hurt cause I didn't invited her to go with me.
Was I the asshole for not inviting her? Will I be the asshole if I don't forgive her? 