Hello and really moved by your experience. You must be feeling that all good deeds are reciprocated with bad ones right now. I had a similar kind of situation to tackle a little while ago(I still haven’t tackled it) where I went from being the most important person in her life to an unwanted presence who kept badgering her in just 1 one week. The first thing you need to realize is that you have no business being in a place where your presence is not warranted or appreciated for that matter. Secondly, For you to have developed feeling for him is very normal considering the sheer quantity and quality of the time you spent interacting with him. And him being flirtatious made things even more easy for you to fall from him.
Now lending a shoulder to cry after a break-up or any other loss is the easiest way to get close to a person as he/she is most vulnerable then and naturally you got to get close to him after his break up. But obviously your efforts efforts were in vain as he never had moved on from his ex. It was here that you should’ve have stopped yourself but you couldn’t.
But now what’s done is done and you just have to get over things and you MUST block him from whatever social media you’re using, contact list, whatsapp etc. That’s the only way you can move forward because those archive messages are a major impediment to your path to freedom. Hang out with your pals(Single one preferably), try enjoying small things in life and there is no better healer than time. It’s all a passing phase and remember THIS TOO SHALL PASS and you are bound to meet someone better, someone who appreciates your efforts to make them feel better and will reciprocate your deeds and feelings the right way.
Good luck.