Why did he left me for another girl?

My boyfriend left me for another girl actually he secretly started flirting with her when he was with me that is considered cheating right? I was so loyal to him when i was with him, our affair lasted for 1.5 years. by the way even he is not mine anymore im still loyal to him i dnt even talk to any guys i stoped talking to guys interacting with guys since i dated him. I always have to go to classes where there are plenty of boys but i dont even look at them. I still can't understand the reason why he left me i truly loved him and he knows it he also said a week before officially dating his new girlfriend that he won't find someone like me in his entire life time again and there's no one matches to me in the whole world but he still can't come back to me he is stucked. I let him go but i need the reason he didn't mention it. Is she prettier than me? she maybe pretty but im not ugly too not being bourstful lots of girls compliment me as their 1st impression and many boys many means a lot have crush on me seeing me pretty, they are like im every boys dream girl. Does she have a better body than me? She is a so skinny literally unhealthy skinny im not discriminating her and i usually have the body according to worldwide beauty standards again not criticizing her i just want the reason why he chosed her over me. And educationally wise after few years im becoming a doctor and her she even failed the easiest exam usually people with lowest iq levels get even a pass in here. Character wise she has a really bad character had multiple relationships and even has cheated in those affairs, eveybody says that she is a h*e. She also new that he had a girlfriend when she also flirted with him. And all his friends say leaving me for is the dumbest thing he did and someday he'll regret it and come back to me. They say that he left a diamond in search of gold. I'm suffering a lot here even its not my loss its been months i still can't get over him. Im planning not do date anyone ever.

Updates
7 mo
guys dont be harsh i asked you guys if any of you had faced or facing a similar situation so you can give a guess about why did he left me, since i dont know the reason i feel guilty if something is wrong with me. Lately i got diagnosed with epilepsy too. So im just so fed up with everything dealing with health, medical entrance exam and this breakup. Pls i need some advic
Why did he left me for another girl?
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