<human>: I'm planning on breaking up with my boyfriend of 1 1/2 years next week, after his birthday on Friday. He's not expecting it, we're not fighting, and it's not his fault, but we're two hours long distance and I'm interested in someone else. I haven't cheated but I'm not in love with my boyfriend as I should be.

My problem is that he's planning a big group outing in a major city nearby for his birthday. He's extremely excited, he's been looking forward to this for a long time. I helped organize it but I'd feel terrible posing for pictures/celebrating/having sex with him knowing that I'm planning on leaving him. I originally told him that it would be too difficult for me to make the trip into the city but I'd visit him later on. He was upset but agreed. 

Now, however, his best friend has offered to pick me up and I can't come up with another reason not to go. So, should I just break it off before his birthday (he would be devastated, especially knowing there's someone else), tell his best friend and hope he'll help me cover without telling, or suck it up and party/take pictures/sleep with my boyfriend? Or something else?

Can you condense the text into three sentences or less?
<bot>: planning on leaving my boyfriend after his birthday this week, would feel terrible celebrating with him. Break up now and ruin his birthday or just keep quiet?