my ex got a new girlfriend 2 years after we broke up and im still heartbroken help

Its been 2 years and although the feeling of loving him is nummed down, like I dont think about him constantly and I dont cry when I think about him, but i do still have some sort of feelings for him. I still feel it was my fault (he left me after cheating on me and falling in love with the girl he was cheating on me with ) im suffering from crippling low self esteem and doubt I even have a new boyfriend and I love him but im terrified of him leaving me or cheating on me. I hadnt thought about my ex for months so much has changed in my life and im in a much happier place im finally at uni, ive got a good group of friends a perfect boyfriend yet when I saw on facebook today my ex had a new girlfriend all the feelings from 2 years ago have come back, I feel sad and I dont know why, I feel like ive just broken up for him and dont understand why i feel this way. I shouldnt care yet im devestated, I dont know if its because this is his first proper girlfriend since me but i feel awful for feeling liek this, i feel like im betraying my current boyfriend for even feeling any sort of sadness at my ex and feel like a stupid little girl, i just need some advice really maybe amn answer as to why i feel like this

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