Jemima, it's so easy to take his excuses when he does come home, he will be remorseful, never do this to you again...and you'll believe him. Not because you DO believe him, but you want to. You want to believe that you can have the perfect life together, you want to be happy, excitedly looking forward to your wedding, your future, your hopes and dreams together as a couple. The thought of tearing all that apart and starting again is terrifying and it's easier to stick to the fairytale and accept his apology.
Till next time. And the next time. And then the time after that.
No one here is telling you to LTB for any reason other than we've most likely already been there. It doesn't get better. He won't stop. You have to accept it, and the sooner, the better, because one day it will come crashing.
I remember standing at the window of our flat, watching the train station across the road for the last train coming in on a Friday night. If he got off it, I knew he'd probably be wrecked, cause a bit of a scene, but at least he was home. If he didn't, it could be Monday morning before I seen or heard from him again. You don't even want to know how many weekends I stood at that window watching that train. How many weekends of broken sleep, worry, my phone surgically attached to my hand in case he deemed me important enough to contact. I was also with him 5 years in total, although I was 19 when I met him. It took me 2 years to leave him. 2 fucking awful, horrible years that are worthy of a whole thread of their own.
Reading your thread has brought those horrible, sick, worried feelings right back. I wouldn't ever, ever, wish to be back there again.
Please don't waste any more time. Finish it now. It will be hard but in a month, 6 months, a year, you'll look back and be thankful you did it. That first step is the hardest, but every one after that is nothing compared to the first.