Idk what to think anymore

Just need a little advice.. my heart and my brain are overloaded with my own feelings and I just need someone else's outside advice... i have been with my fiance for 17 years.. about 2 years ago he left me at home said he had to go find his wallet.. he shut his phone off.. returned about 4 hours later.. said he went fishing. When I got in the car that morning there was a woman's headband on the dash of the car. He blamed it on everyone but him.. said he didn't know how it got there. I gave him the benefit of the doubt even though i knew better... well he played innocent for over a year.. a female who had been a mutual friend or so I thought.. messaged him and asked if he thought the baby she just had was his... said she didn't remember when they did it but it would have to be a paternity test. I confronted him and he admitted to sleeping with her that night.. but that is it. He wont tell me anything else.. wont tell me how long they talked or what led up to it.. if they talked after... am I wrong for wanting these answers? He says im sick and twisted for wanting to kno.. but i can't let it go.. hes cheated in the past but this time its different.. she came to my house in between the time it happened and b4 i found out it was her... they were friendly and such until I found out.. now he refuses to talk about her.. please help.. i just feel like I will never feel the same for.him.. unless he is an open book.. i had no clue he was talking to anyone.. let alone having unprotected sex with someone I trusted...