I'm confused and need a guy's point of view on this please?

To put things out there, I'm a bisexual male and recently had what I thought was the greatest boyfriend ever even though it was a long distance relationship. we cliqued, had similar interests, talked for hours, and even made plans to meet over the summer. But there were some worries that I didn't see. One worry that I should have seen was that he actually came tome and told me that there was another guy at his job who constantly asked to date him and my stress levels rose, but he reassured me by saying that he declined him and things continued normally. However, he started becoming distant and more 'busier' than usual and I asked him about it. He basically told me that he was busy playing games or that his job has him busy--even told me that he purposely occupies himself where he wouldn't have to talk to me that much. I was honestly wondering what the heck was going on! Someone that I was so close to and cliqued with starting to act like this?

One day I decided to ask my friend about him who is roommates with my ex and he got angry at me since he assumed that I only talked to him to talk to my boyfriend, which I didn't though it made me feel down. We ended up arguing and he blurted out that my ex was cheating by dating someone in their state of California. My stress rose, I was shaky, and somewhat panicked, but I calmed down since I figured that it was probably a lie due to us arguing...though a part of me wondered why he would blurt out something like that. I confront my ex about it calmly and he denies it and says that he's only dating me and would never do something like that to me and things continued and we seemed to be close like when we first dated (though he WAS actually dating someone else behind my back, which I'll be getting into). I suddenly don't see him or get contact from him for three days and my friend calmly tells me that my ex is alright since I worried about him, but that he was in the living room with his 'boyfriend' and that he didn't want to tell me what they were doing (which meant they could have been having sex since he usually tells me when someone is making out or something else along those lines). Even though it was calm and since I didn't know he was actually cheating, I asked to talk to my ex.

We begin to talk and I ask him if he's dating someone else and he plays it off by saying that he's dating me. But strangely I persist and ask why his friend would say that he's in the living room with someone and him being gone for three days without contact is highly suspicious. Once again he plays it off, says that he's in the living room but not with anyone, and so what if he was offline. Eventually he gives in and tells the truth though, saying that I got 'boring' and turned into some whiny and submissive person, that I'm not the right guy for him, and that his new guy made him happier than I did. I know that he's on the rebound from jumping from one relationship to the next and probably kept me around just in case things didn't work out, but I'm just completely confused as to why he didn't just come and communicate with me about this problem instead of doing these backstabbing and heartless actions. Not to mention that I'm 20 and he's 22, so i'm surprised by this immaturity as well. Sorry about this being long as well, but I really need some assistance on this...