How do i stop worrying about if my ex is sleeping with someone else?

my wife and i just broke uo after 15 yrs .i love her more than my heart needs blood.she seems to have an 18 yr old friend that comes over to see her.spends time with her alone.she says nothing is going on but others tell me different.i dont know but when she calls everyday of the week than all of a sudden she stops on the weekend.she came over today and she told me she loves me and slept with me.when my our son got ready to take her back home she started yelling like i did something wrong.this gives me bad signals.i take it like she tells me what i want to hear then when its time to go home it seems like she has to be mad at me before she gos for some reason.this is why i think the way i do.am i wrong for getting that feeling?if she is sleeping with this kid i think its easier for he to **** around if she mad.then all i do is worry.cant eat,cant sleep all i do is worry.