One foot in, one foot out of my relationship.

I have been dating my boyfriend on and off for about 4 years but this past year has been solid. He has messed up multiple times, probably over 20 and I've given him a chance each time. He had been Facebook messaging women on a sport fake account for a while and i found out about it a few times and each time he would say he wouldn't do it again. he made so many dating accounts online and i found all of those too. I am going to be away on vacation with family for two weeks on dec 12th and don't know what to do since i obviously don't trust him. I have tried telling him that maybe we need to be separated because he needs to get the teenage boy out of his system without a girlfriend telling him no. I feel like since i say no and don't support him doing things like online dating etc, it makes him want to do them more. I just don't want to come home knowing that he probably messed up again. I love him very much, we are high school sweethearts but I just don't feel the spark anymore. I have no self confidence left, and i am constantly doing for him while i get nothing in return. I have every reason to leave but continue to make excuses to stay every damn time. I really need some help with this before I go crazy. I have done nothing wrong and have remained 100% faithful, and am turning to strangers for advice because my friends are limited (thanks to him). I'm begging for some genuine, open hearted advice. Thank you so much :/

(p.s: we are both 20 years old)