could do with some advice if possible

hi guys i will try to make this as short as possible. i was with my ex for 2.5 years everything was amazing best friends did everything together etc, she had an 8 year old little lad from her last relationship took him to school and all the normal stuff and even though he was not mine i loved him as much as my girl, his dad was an ass used to fail to pick him up was always out drinking smoking weed etc just a total ass, now when i first met my ex i was her shoulder to cry on about her ex being awful and i backed her up all the time, when she was with him she was depressed taking drugs and very over weight, she left him and got with me 6 months after they split. in jan of this year we had a bit of a fight as i was feeling a bit down as my dad passed a few years ago over xmas and i was a bit snappy etc. now she told me to go and blocked me on all social media etc, today i got a like on one of my facebook pictures off her ex so i thought what the hell and low and behold they are back together which has left me heart broke again as i can not believe she went back to the one person who she hated and i backed her up so much for her little lad. oh and when she left me she said thanks for fixing me you will always be the love of my life and soul mate, i asked to get another chance to put things rite and she said it was to much me going on about how her kids dad all the time? but it was her that planted the seed for this. im in my 30s by the way this girl has left me devastated and miss her so much. i feel like some dick is having my life now and im left with nothing. i text her and said you could of let me know you was back with that ass hole instead of me finding out like this she said im so sorry just live life and be happy, she has left be baffled as to how she could go back to someone like that?? ok its the kids dad and she said she wants a family but jeez i was building it, so i guess my question is has anyone else been through anything similar to this?

thanks in advance guys