Re: So, i found out he was cheating…
What difference does it make? Is knowing “WHY” going to make you feel any better? I bet you he also wonders why you did many of the things you did (for example: stalking him on whatsapp!) but you don’t see him running after you with silly tantrums and messages do you?
No doubt what he did was wrong. However, personally, I’m more disturbed by the fact that you used your sister’s phone to “check up” on him AFTER breaking up with him! 
And then messaged him about it! 
Quite frankly, your lack of self-respect and self-confidence is sad.
While I do not condone his behavior at all…even as a woman…based on many of the posts you’ve written about the way YOU behaved in this situation…sadly I can’t say that I blame him for not wanting to continue this relationship. 