does this constitute as cheating?

(disclaimer: i'm 18) i started dating my boyfriend (now ex-bf) back in early may. we were really good friends before we started dating. he asked me out back in february, but i rejected because i feared our friendship would be ruined. he asked me out again early may and i said yes; because he was crushing on me for 11 months. we had a sweet, innocent relationship. we did sext here and there, but it wasn't a constant thing. we only did that to add some spark to the relationship. we went on dates and had the time of our lives. i thought we was really truly the one... until one night. we both attended a party in late may and my ex boyfriends old crush attended as well. sometime throughout the night, i noticed my ex and the girl (his ex-crush) were missing so i started panicking a little bit. i didn't think much of it though because i trusted him and the girl. today, i found out that my ex tried to hook up with the girl in a back room.  fast forward to last wednesday night. i looked into his phone bc i started getting suspicious of him. he never wanted to text me, call me, hang out with me, or even be around me. i discovered that he was flirting with the girl and he was planning on dumping me. so before he could, i dumped him. my heart is shattered. this is my 3rd relationship. i was so committed to him. now the situation is even worse because it hasn't even been a whole week since this all went down and he's ALREADY dating the girl from the party. and i'm still in shock about this. i want to put everything behind us and move on, but it's so hard. can i have some tips?