What do you think the reason for this was?

my partner of 8 yrs nd i just stay at home now.we are stuck in rut,and he sys he doesn't fancy me anymore,when I am slimmer and better than before.he says it is not looks,the relationship sucks.on saturday night he rgued to get away from me,he went to his sisters house,but had a change of heart and came home.what is going on here.?he has never cheated before,and leaves his phone around so i know he is not cheating now.what do i do?