So confused

Hi everyone,Sorry if this isn't the right place, I'm not sure where else to go. Also a throw away account due to anonymity.So I always considered myself as straight, by no I've ever been a player, but I've been with a few girls and enjoyed my time with them. I'm now currently in a relationship with my current girlfriend, we've just celebrated our 3 year anniversary.however recently I've noticed that I'm no longer excited for sex. Whenever we try to, I get an erection, but when the time comes for it I go soft and can't get it back up.I've noticed that I'm far more sexually excited masturbating to gay porn, and often have gay sex fantasies. It's the only way that I get hard now. It's even resulted in me downloading gay hook ups apps to talk and flirt with other men, although I've done nothing with anybody.I'm racked with guilt, I don't know what to think anymore.Can anybody relate?