How do i begin to tell my bf

Of 8 yrs and have two kids with that i cant be with him bcuz i dnt trust him?? I know he cheated and all that was three yrs ago the trust just isn't rebuilding all hes going to say is that he isnt doing anythjg wrong and i haven t caught him doing anythjng but its this unfinished feeling i always get right when he walks out the door.. I mean how do i start this convo out of no where i dnt want to yell i want him to know im serious and just dnt feel the trust...