SUBREDDIT: r/relationship_advice
TITLE: I think my ex cheated and then left me for someone else, then possibly cheated on his new partner. I want closure.
POST: I hope this post is okay, it's my first time posting here.

First I wanna say, I don't and won't get back with him, the relationship was mentally and emotionally crippling on me.

Anywho, I have very, very strong proof that my ex cheated on me, left me for another girl, and then cheated on his new girlfriend.

So myself (17/MTF) and my ex (19/FTM) were together until about mid December last year. After he left me, I saw his close female friend who I know was really into him, at add him as her boyfriend on Facebook and posted about it on her Instagram. So I confronted him about it, but he said it was just so the girls creepy friend would leave her alone. Before he moved away, we hooked up once, on the thought that we were going to get back together. Throughout the months I've seen posts by her talking about her boyfriend, and tagging him in the posts. Also her getting a promise ring from him, though he denied it and said it was just her obsession with him.

Now fast forward to about two months ago (give or take, I'm awful with time passed). He comes to me after I've emotionally moved on saying he wants to get back together. So we chat a bunch and then drift apart as I realized how much I disliked about him. Meanwhile, during all this, the relationship status stayed the same, and the girl was still posting stuff about him. NOW I go on his Instagram and see that he posted a song with his caption "this song reminds me of my girlfriend, @herusername". Meanwhile, on her Instagram, she posted a picture of what looks like to be flowers and balloons, captioned "my boyfriend sent me this for our five months @myexsusrename". But I guarantee he will deny it again. 

So, am I just a gullible idiot, hoping I wasn't actually cheated on. Or is there something that could possibly explain that everything is just a big coincidence?
TL;DR: 
i have strong evidence that my ex started dating someone before they even broke up with me, and to this day the evidence keeps getting stronger. Was i cheated on, or am I missing something?