Why do we stay together?

We have been married for 30 years, 20 years ago i screwed up big time .I fooled around on my wife not smart but i did it cant take it back, the really bad part is i got HIV, and gave it to her. We are ok for now good days bad days . I guess i am lucky she didnt kick my ass out ,we have nothing to do with each other , we have not had sex in ten years I still love her but i cant fight about it any more .The kids are growen 26,23,19 all know whats up Dont know what to do.?