My boyfriend just confessed to me that he slept with 2 other girls?

Last year my boyfriend and broke up.......we told each other we was going to move on.... he went out with someone so did i..........then 3 months ago we broke up again.............i gave my virginity to him and he to me. we told each other that we werent going to sleep with anyone untill we were for sure. after we made love like 2 days ago i kept insiting that i had a bad feeling that he was keeping something from me. at first when we got back together he promised me that nothing happend....but it was a lie...........he confessed to me that last year he had with the girl he was with...and then also the girl he was with over the past 3 months.... i get it that we werent together and i dont own him but. i forgave him.......and we are still together.....but everytime i want to talk about it he doesnt he told me that he wants to keep it in the past....i feel like as his girlfriend i have the right to know things but i let him be. What can i do to get over that. i want to be happy like we were before i found all this out...........he comes over every morning takes a shower with me and sometimes makes love to b4 he has to go to work...........there im happy but once he leaves i feel like somethings going to happen i feel insecure. ..... i dont want to be that way. i love him and i know that he loves me but that just kills me inside sometimes.... how can i forget?