I don't understand my ex?

so i dated my ex fiance for 4years off and on. we grew up together and he was always my best friend even when we werent together. i broke up with him the very last time because i needed time to focus on school but then 2 months after i broke up with him, my dad ped away and he was the first person i called right when it happened because he was extremly close to him and my dad called him his son(which he is the 1st boyfriend i ever brought around my dad and my knew him since we were young). he went to my dads funeral and was there for me and that day i had to give him back my engagment ring because my dad wanted me to and i promised him i would(my dad just thought it was a promise ring) but my ex wanted me to keep it but we were standing right in front of my dad and saying our goodbyes and i told him that he wanted me to give it back to him n i promised him so i had to and he understood but he told me he would keep it until we got back together and would give it back to me. i was so heartbroken and sad and upset that i seen my dad p away in front of me that i was so depressed that i pushed everyone i knew and loved away and i locked myself in my room and didnt talk to anyone for a long time. my ex would always leave me comments and tell me that he still loves me and he always will love me and he cant wait for us to get back together but i kept ignoring everysingle comment he left me because i didnt care about love or him or anyone at that point. n its been 9 months since my dads funeral and at this time i fond out he is engaged to a girl he met on the internet and only dated for 2 months and gave her MY engagment ring and moved in with her and her family and we didnt talk at all for those 9 months. i seen him 2 days ago, the first time i seen him since my dads funeral which has been a year and 3 months. he had his new fiance with him and they kept walking right in front of me holding hands and smiling at eachother, like i cared because honestly, this girl is hella ugly. no joke(all his/my friends said that he is much better being with me than her, which obviously made me feel pretty good about myself.lol) they were near me the whole entire time i was at this place and i was trying to be friendly and smile and i wanted to go intoduce myself to his fiance to be nice but i thought twice about that cuz that would be a bad idea and then i would probably be in jail right now.lol. but i just sat where i was, minding my own buisness, watching my "friend" bowl because i went there to watch him. i glanced over to where my ex n his fiance were and he wasnt near her at all which is the funny part. (whenever we bowled together or he had a tourny i would go and watch him and he would be alllll over me, he couldnt keep his hands off me and kissed me all the time which he wasnt doing that to his finace at all) and when he was sittin and his finace wasnt looking, i caugh him so many times looking at me and turn away really fast. i was waiting for him to say hi to me because we had so much history together n grew up together that i would at least think he would say hi but i waited for 4 hours and i never got a "hi" so when he walked by me he looked at me and i said hi first and he just walked right past me and waved his hand without looking at me and didnt say a word. he has NEVER acted like this towards me in our life. yea hes dated other girls and i dated other guys when we werent together but we were always best friends and friendly toward eachother and he messaged me about 2 weeks before i seen him and asked how i was and if i was talking to his bro in law (which i had no idea who he was talking about until i actually started talkin to him as friends) and he was extremely nice to me until i seen him in person. i dont understand him at all. i know i was his first and his first love, and he was my first love but i dont understand why he acted like that to me at all. it caught me hella offgard cuz his dad even talked to me and his dad never liked me(well not when we kept breaking up because my ex would go all sucidal on me). but i dont know why he would be looking at me and then not showing affection towards his girl the way he ALWAYS did towards me in front of me because i know thats how we always were in front of both of our exs together, we would be all over eachother because we were so in love that we didnt care what ppl thought. but y would he be like this when he sees me in person???