I really could do with some advice please!!:(?

ok i have been with my partner for 17 years, not long before we had our children, i had a very brief affair, my partner kind of knew but i continued to lie about it. knowbody what so ever knew. for the past 11 years it has been an ongoing thing here and there (the questions and doubt) it all came out 10 months ago the truth, as he couldn't let it go and i could see how much it was hurting him the truth had to come out. Since him finding out, it has been a constant hill to climb, we love each other so much, but he cant put it aside. I know what I did was wrong i was 21 had no family around i could probably say no friends either as i lost contact with them all because my partner was (is) my life and i didnt really make time for anyone else.Some days are ok but others are very very emotional. I'm not a tart i dont sleep around, i dont even really understand how i let it happen, theres so much more but not enough space, i think i need to vent as he has just walked out :(