Fear of being hurt cheated on .. HELP

An issue Ive been dealing with is the fear of being hurt cheated on ... Ive been cheated on twice and that was enough to leave me with fears and triggers ... my mind and heart are always battling each other .. when i try and think positive my mind tells me the opposite and I go into fear mode ... Im in a relationship now for 6 yrs ... but theres times where I need reassurance and i check everything to see hes not doing anything bad .. its driving me insane I want to get past these feelings and fears but I cant ... the fear of being hurt or deceived scares me ...