Why was I the sidechick? How can I move on?

I'm going to try to make this as short as possible.
I am 21 years old and I fell in ove with this guy who I met around a year ago. When I met him he told me he was single and we started dating and became exclusive. We had a relationship for 8 months and he was my first boyfriend. He was older than me (15 years older) but I thought age was not a problem because we got along great and he would always tell me I was the love of his life.
Throughout our relationship he never invited me to his place, which I found odd. After two months of knowing him and being together I finally confronted him about it. He confessed he was still living with his ex and he made a whole story about how they couldn't sell the house etc. Of course that made me start to doubt his intentions and whether he was still with her, but everytime I confronted him about it he would make me feel bad and tell me that if i didn't trust him it would never work etc. One time he even went as far as to tell me to ask his ex myself if I didn't believe him, which of course I didn't do because I believed that he would never say such a thing if it weren't true that they were no longer together.
After some rocky months he finally invited me to his place when I gave him an ultimatum and this calmed my insecurities. He met my family, he was always present in my life and always assured me I was the only one for him and that I was the love of his life. After 7 months of being together he finally moved out from his house and even moved in to the city where I live. He even asked me to move in with him but of course I said no. However, he started to act distant again and he would break my heart many times so I decided to leave him. We continued to stay in contact and he said he wanted me back etc untill I found out that from the 8 months we were together...7 months of those he was with his ex. I also found out that the same month that he was also dating another girl and now they are together.
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And i just want to add that now i feel like shit. I feel like he never loved me and I literally gave him everything. I always always put him first and I truly believed he was the love of my life. Now Its been a month and I'm still hurting and have so many doubts, while he is happy with the other girl he cheated on me with. I just don't know how i will ever trust again and I feel like I have been played and like im the stupidest girl on planet earth
Why was I the sidechick? How can I move on?
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