I Made The Biggest Mistake

My girlfriend and I have been together going on 5 years this coming May. I am 22 and she is 21. Things the last few months have been pretty hectic some days, others are great. It has basically come down to this. I haven't proposed to her yet. We have been living together in our own rental house since 2011, and have been close ever since we got together. I can only remember maybe 5 days that we didn't get to see each other. My younger brother who is 18 proposed to his girlfriend, and everyone makes comments to us on why we aren't engaged yet which puts pressure on her which entails putting pressure on me. I know for a fact that she is the love of my life. She understands me, we get along really well, share the same interests, laugh with each other, smile, etc. I don't have a reason for why I haven't proposed yet. I want to get her a nice ring and try and save up money, but I feel it's been impossible to save any money at all for one. We are trying to pay off minimal credit card debt, and I don't want to add more onto it until we get that under control first.

Here is the main part of my story. At the beginning of this year, we started going to a Planet Fitness that opened up in our mall. We signed up and starting using it very frequently (4-5 times a week). One thing led to another, and I got into the habit of really enjoying working out together. I ended up downloading a fitness app on my phone and using it to log my calories. The site has a friend list type thing like Facebook so you can see people post things and can message them as well. I work an office job. My girlfriend works a nursing job. We go to the gym right after work. We make dinner, shower, and head to bed. She pretty much falls asleep as soon as we lay down to go to bed. Our sex life is maybe once a week if that, which isn't enough for me. I am 22 years old, and I am sorry to say it but I am horny, especially at night. One thing led to another on that fitness app and I interacted with 2 girls on there. I would talk to them occasionally when I would check my phone during work and after work when we were settled for bed. We ended up getting into some sort of fantasy idea that was a turn on. It ended up leading to me exposing myself and sending pictures back and forth to the point of masturbation. I have done it a few times with 2 women on there. It wasn't a real life experience. I never wanted to make it a real life experience. It would pretty much me using my imagination to get myself off and get my fix for the night.

My girlfriend ended up finding out about it this past Sunday when I left the messages up on my phone. I was drinking that night and must have fell asleep looking at if after she fell asleep. When I first woke up that was the first thing she said. She confronted and said wtf is this. I knew it was going to get pretty bad. I basically told her IDK and don't know why I did it. I guess it's the fact that I want more intimacy with her and that it's not normal for us being so young to not have a healthy sex life. I told her I was in the wrong by not talking to her about it and feel terrible about it. I won't do it again after seeing the pain that she has gone through from it. I told her I would never physically cheat on her, but she still claims this is cheating and on the same line as physically doing it.

The last few days have been really weird. We have had sex two times since it has happened even though she says we aren't together and are single. She has gone out to the bar with her friends and of course they are on her side and putting things into her head to really make her confused. It's just weird. We still act the same and cuddle and stuff but she says what are you doing when I do it and still goes along with it. What do you guys think I should do? I completely admitted to be in the wrong and blame myself for the situation we are in. There was a slight reason I did it though. To make things even more interesting, she said why couldn't it have just been porn you wacked off to? I wouldn't have cared if you did that. She cares because she thinks I wanted to be a part of these people's lives on the fitness site, which isn't true. It was mainly someone to talk to which led to other things...

I feel terrible about what I did and want to fix things. Seeing her cry last night broke my heart. I hurt her and I am the one person that shouldn't ever put her through what I did. I love her more than anything and feel she is the only woman I want to spend my life with. I want to marry her soon one day and had plans to coming up this spring when I take her horse back riding. What do you guys think I should do? I am all over the place and hope this topic makes some sense and you guys can give me advice. Thanks!