Previously unfaithful boyfriend testing my ability to remain calm. Help?

My boyfriend of seven months, (both nineteen years old) cheated on me with his ex-girlfriend about a month ago. We have discussed the issue at length and I felt comfortable and happy continuing in a relationship with him.

However, before he had cheated on me, he had met up with an old friend while out on a date with me. A few days later he informed me that he had went out to dinner with this girl shortly after seeing her again at our date. It rubbed me the wrong way at the time, just a bit, but I shrugged it off.

Now it is about two weeks after having learnt that he cheated on me with his ex. He has just informed me via text, after my asking, that this evening he is out with this old friend at a party. This is seriously pressing my 'freak out' button. Especially when he said that he would try to have fun but he wasn't really into the party. I can't help but wonder why he would go then? I'm sure this issue has plenty to do with his recent actions, and am unable to really look at this situation clearly and examine whether this is or isn't appropriate on his part. Of course, going to a party with an old friend shouldn't bother me. Right? I feel like I'm going to explode over what really shouldn't bother me. How can I feel better about this? How do I stop the urge to send 'do you like this girl? are you sleeping with her?' texts?

TL;DR: Boyfriend who recently cheated on me is currently out at a party with an old friend. Am freaking out and am not sure what to do/how to feel.