How to get over the only man I have been with for the past 7 years that i have 2 children with?

I have been with my boyfriend/fiance for 7 years. We have two children together and we make plans to marry but our relationship goes through so many ups and downs it ridiculous. We have been living apart for the past 6 months (still together) and it felt so much better not arguing and fighting every other day. Even though I missed him most of the time it felt good not to have an argument, sleep on the couch and then eventually make up. So tomorrow we are moving back in together and in February I plan on moving out and getting my own place. If their is anyone out there that has been in my situation can you give me some opinions. How can I really take that big step and move on with my life? How can I take that step and don't allow him to wedge his way back in...pretty much come over whenever he likes, spend the night, think he's gonna get "some". Any answers helpful. Thanks