I feel so horrible and I do take partial responsibility for what happened.

Thing is, my ex-boyfriend who I had broken up with 4 years ago who went into the U.S. armed forces was finally visiting town again. We hadn't seen each other in 3 years and talked sporadically via email/text/cell.
I had gotten over this guy, maybe not completely, but was over him (who I had only dated for 1 month).

My current boyfriend, I've been dating for over a year. He is amazing and I really do love him very much.

What happened was that while we were hanging out at Starbucks, he kissed me. I pushed him off and reminded him I had a boyfriend. But then, later, he kissed me again. He wouldn't let me break off the kiss and I ended up just giving in and kissed him back, going a little further as well. I feel horrid now... I'm so stupid.

What should I do? Should I tell my boyfriend? He's not the kind that would forgive such a stupid transgression. Not to mention I don't want to cause him any heart break.

Please help.