I know its toxic but i can’t leave..

So i’ve been with my fiance for about 3 1/2 years now. We started off pretty good no problems but we started moving a little too fast, we got comfortable with eachother as soon as we clicked... we fell in love.. then thats where the rockiness starts to fall on our road... i catch him texting females ans he asmitted to taking a coworker home and kissing her, i forgave him because little did he know I technically did the same. So we moved on and tried to be better for each other. Time rolls and we’re at our year mark of dating and literally 1 week before he proposes i caught him texting some bitch that took his order at fucking taco bell...... i move on i forgive our relationship starting going downhill.. i had my share of toxic mistakes because i deal with anxiety and a bit of BPD (borderline personality disorder) we took a break for a while on my birthday month and i caught him trying to text other girls so i played the same game and texted my ex and we started hanging out and then his BM found out and texted my fiance about it lol and he got mad and i said you’re the one that broke up with me so i didnt do anything wrong, i was single??? So he took me back lol. But his anger started taking a sinister turn shortly after..... he started getting into my head, he pushes me, punches me, spits on me, throws my belongings at me, he even pretend to calm down just to gain my trust and get closer just so he can grab me by my hair to drag me out of my way.... he’ll kick me..... say all the horrible things he knows will break my heart.... like calling me a bitch... ungrateful that im a pig and no one will ever want me... and that im just gonna get back on my knees and beg him to stay..... idk what to do.. this has become something i deal with on a regular basis and im afraid he’ll lash out at anytime and it’s honestly scary and heart breaking because his dad did the same to his mom and i was hoping he didnt end up like his father..... sorry this was kind of long ladys, if you’ve been in this situation how did you get the courage to leave....?