Advice Regarding Mum rights?

Urghhhh! I really really never wanted to find myself on this situation but here I am! I have been with my fiancée 8 years now, we have two beautiful children LG nearly 4 and a lil boy 8 months and I am heartbroken for them that this may happen, however me and my fiancée have been really in a bad place for a long time, we've been to therapy, it hasn't helped! So I just wanted to see if anyone could she's some light on what would happen next if we broke up! He gets really nasty re the kids when ever it has come up that we may finish this - so I just want some advice if anyone knows whats what's when it come to this. We are not married, we recently bought a house together so jointly own - i dont want to go to a family court as I've heard they are awful but I know he will get nasty, so I just want to know what's the norm I'm this situation re kids custody etc. I am being super amicable about things he is not! Help please if anyone knows xxx Thank you X heartbroken for my children I don't want to do this for the effect it mY have long term but I can't carry on like this