Why do we stay together?

I know i asked this question before but i needto for my wife, we have been married for 30 years now . Twenty years ago i stepped out on my wife and if that wasnt bad enough really bad thing is i got HIV from the encounter and gave it to her I live withit every day the gulit is heavy and i have no one to blame but my self. She a great women and i hurt her to the core . We have not had sex for i think 10 years now and i really dont blame her its my falt yet we stay together. We have 3 kids 2 sons 1 daughter .They are all ok we have had them checked a few time through the years. The kids are older now 26,23, 19. What do you think keeps us togehter ? I know we still love each other and care when she gets sick i get so mad at my self because i know its my falt.What do you think? Thanks for your time.