How do I leave my relationship?

I need help! I have been with my boyfriend since I was 16 I’m 23 now we share 3 kids together. We’ve been having issues trouble staying together for the past year. I believe the only reason it “works” is because I make it work. The past 2 weeks have been super rocky hes left once but came back. I got the urge to go through his phone over the weekend and found 3 blocked numbers all 3 were text now #s. So I sent them to my friend and she called them. Only one person answered and it was a female. So my friend calls me on 3 way with this girl so I can hear for myself. She said she is in sex work and if my man has her # blocked more then likely he’s been seeing her and paying for sex. She couldn’t confirm because I do not have any hard proof just a blocked number. At the end of the conversation she apologized and let me know she’s only 18! He’s 26! So I’m pretty shook up at this point. I woke him up ask him about all these accusations and of course he denies it. I made him leave he came right back that night. I need help to stick to making him leave and believing he will not change. This is NOT the first time he’s been caught paying women for sex. My self respect is just gone I feel so dumb worthless & used! I just don’t know where to start. HowTo be on my own and be okay. We just had a little girl 6 months ago i do not want her to tolerate the things I do. I KNOW I need to RUN and never look back. Why is it so hard to leave?! Any and all advice is needed