what to do?

I honestly don’t know what/how to feel. Me and my SO have been together for 4 years, and everything has been great. but recently I feel like we haven’t been hitting it off. we don’t have sex, go out, or anything anymore and i don’t know if it’s because we live together or what. so 2 weeks ago I went to get my hair done and i was talking to my hair stylist and she went through the same thing but she ended things with her bf. and I feel like i want to do the same cause I don’t feel happy or wanted. well on that day some guy I used to talk to like4 years ago hit me up. and we ended up meeting up that night and we had sex. I cheated on my bf and idk what to do. the other guy is engaged, and I know i fucked up. but i haven’t felt good for awhile until that night. now i’m just feeling guilty and just don’t know how to tell my so. our anniversary is in a week and this is just bad timing. he doesn’t deserve this, he’s such a great guy. I just haven’t been feeling “us” lately. and i know he hasn’t too but we don’t communicate anymore. and i hate to hide this from him. i’ve never cheated before, and i’m scared to tell him. he deserve the right to know even though it will hurt him.