Becoming "just Friends"

So most of my relationships have ended with my girlfriend saying she wanted to stay friends after breaking up. Now Im intuitive enough to know when my girlfriend is being sincere or not, so Ive said on all accounts "I'll try, but it'll take time". Now I really did need time lol, I wasnt being a **** or anything. See I really miss my most recent ex, I think we were soul mates, but Ive been thinking about what I said before and I realized with my other exes we didnt end up becoming friends again, we're just nothing. Throwing aside my lingering feelings for my ex, I really want to be at least friends. I mean we've been through a lot, and even though she hurt me I still trust her, kinda. Anyway I just wanted to know if anyone else has managed to be good friends with their ex. Can anyone give me some advice as to how I should try and treat her or etc? Cus I wont lie its really awkward between us right now lol.