Ok, just a few observations.
1. You dumped your ex to see the new guy, yet you traveled to see the ex and were gone 3 weeks at the ex's in order to break up in person. If I were the new guy, I would have ended things at this point.
2. Why, if you were interested and dating someone, would you allow some other guy who was obviously interested in you to take you home from a party that you were at till the wee hours of the morning?
3. Why would you even go to this party without the guy you were dating? I guess you just didn't have any idea he would get jealous at all, did you?
4. How in the name of sense should we know why he is doing this? Why don't you ask him why he is doing this? He should know better than we do.