Birthday Text...Lengthy but need help please:(

So my ex broke up with me and told me he needed to work on himself; he felt he didnt make me happy. We were together for a year and a half. Well this girl I was suspicious of during our relationship(the last 2 weeks of it) I found out he was talking to while we were together. Once we broke up he slept with her and anytime I’d catch him with her when we weren’t together, he would deny anything was going on. It took him 2 months to admit that they had slept together and he told me he cares about her and doesn’t know if he sees a future with her but he loves me and sees one with me.
However, in our last conversation about 2 and a half weeks ago he stated he needed time to get over messing things up with me and I needed time to heal and that in a few months this would all be over with(meaning him and I.) He was supposed to call me that night and never did. Found out he stayed the night at that other girls house. I told the girl some information pertaining to her health about my ex and she went off on me. I was pissed at him because I told him he needed to tell her and he said he would but never did. I went into NC at that point.
I am on day 19 of NC. I know I shouldn’t be looking but on Valentines day this girl unfollowed him from instagram and put a picture of her guy best friend saying he always can cheer her up and that he’s cute. However, his work I have to pass to go to my apartment and the next day her vehicle was there; meaning he had drove it to work. Then her birthday was on the 20th and he had liked her picture. But she is the type of person that if she were with him she would post pictures, because she has posted pictures of them before.
The day after her birthday, she completely deleted her instagram; which was so weird. Anyway, his birthday was the 22nd and I caved and sent him “I know we aren’t on good terms but just wanted to wish you happy birthday.” However, he has an iPhone and it went through as a text message. Since we broke up he has neglected paying his phone bill so he could only text when he had WiFi. Now his texting is completely shut off until he pays it. We both have vine accounts, so I sent him a message on that saying “not sure if your phone is off or if you got my text but just wanted to wish you happy birthday.” So far it looks like he has not been on vine to read it. I sent this on Monday. Also on his instagram he has followed 3 or 4 girls with my first name.
The NC is getting harder as I get more into it. I am really struggling. But i never stay cooped up in my apartment, I’ve been working out, reading, writing in a journal and making new friends but I still feel that pain on the inside of missing my other half. Did I mess up on saying happy birthday? Should I wait longer until I do send him that text? I also don’t know when he is going to turn his phone back on. Could he be serious about this girl? Please help!

Did your ex ever say why he thought that he didn’t make you happy? Do you think this is a genuine reason for ending it?