Is it normal to have thoughts

iv been with same girl since i was 17 we both in our 50s now . i honestly tell u i love her and i know she loves me . my life would be complete but for one thing . in our early 20s she had affair . it nearly killed me we split but we did come back to eachother . but still to this day i ask myself how could u do this . shes said she was young n foolish but i still cant understand how n why . what else we met when 16 / 17 we dated n never had sex till we both 19 . yet she met a guy n had sex with in weeks just find it strange . this drunken excuse doesnt work on me . truth is the love of my life let me down n hurt me like no other . yes i forgive but il never forget . and it always be the little thing in my head that the love of my life ruinef the fairytail . i love her always will .. but il never get over what she did am i been wrong