If its eating you inside then here's where you can find out if your guy can be understanding, if you really must tell him say you had blanked it oout because until he reminded you of it you had buried it because it made you feel less then you are tell him that you was pressured and how bad it felt for you and how much you needed to bury it dont say forgot because this trauma you dont forget you bury inside yourself.If your guy is understanding and as in love with you he will know it was against your nature and hopefully be there for you now.However this is a risk and one only you can decide.
We all take baggage into a relationship and it does come out we also learn from previous relationships what / how we will treat and want to be treated by others.
Remember this is a risk for your relationship
A mistake is always forgivable, rarely excusable and always unacceptable.
Robert Fripp