Feeling incredibly guilty and can’t get over it

I'm a 20-year-old guy, and I've been with my girlfriend for almost 3 years. I love her, and throughout our relationship I've always been faithful. I've never cheated, flirted with anyone else, followed someone on instagram, liked womens pictures, or acted on any romantic thoughts.

A few months ago, though, I went through a really confusing period.

I started wondering if I was "wasting" my 20s by being in a long-term relationship. My feelings felt weaker than before, and I genuinely thought I might break up with my girlfriend. I felt confused and started questioning everything. Since we'd been together for so long, ending things felt incredibly difficult.

Around that time I started an internship. On my first day, someone asked if I had a girlfriend, and I immediately said yes.

Later, while talking to a coworker and telling a story I referred to my girlfriend, and instead of saying "my girlfriend," I said something that basically translated to "the person I was with." I think it came out that way because, in my head, I was already considering ending the relationship. I also changed my phone wallpaper from a picture of us because I was thinking we might break up, and I didn't want anyone to see.

There were also a couple of coworkers I briefly found attractive. I had passing thoughts like, "What if I were single?" But that's all they were: thoughts. I never flirted, never tried to get closer to them, never messaged them, never followed them on social media, never liked their pictures, and never gave anyone any reason to think I was available.

Another coworker later asked if I had a girlfriend, and once again I answered yes without hesitation. I never lied about being in a relationship. After I answered, I glanced to the side to see if one of the coworkers I’d had those passing thoughts about was there.
She wasn’t. But even in that moment, I was absolutely sure it wouldn’t have changed anything. If she had been standing right there and asked me herself, my answer would have been exactly the same: “Yes, I have a girlfriend.” I never wanted to pretend I was single.

The night before I planned to break up with my girlfriend, I talked it over with my friends. I was convinced I was going to do it.

Then I saw her.

I couldn't go through with it.

Instead, something just clicked. I realized we had simply been going through a rough patch, and I'd mistaken temporary doubts for falling out of love. Being with her made me realize she's the person I actually want. The desire to experience dating other people in my 20s completely disappeared. I realized I don't want anyone else - I want her.

The very next day I changed my wallpaper back to a picture of her. At work I talked about her proudly. When my coworkers were saying that relationships are only exciting in the beginning, I told them I completely disagreed. I said I'd been with my girlfriend for almost three years and that I was more in love with her than I'd ever been.

The thing is... I still feel incredibly guilty.

I told my girlfriend everything. I told her I had changed my wallpaper because I thought we might break up. I told her I referred to her as “who came with me” and not “my girlfriend” because I was mentally preparing myself for a breakup. I told her I'd had passing thoughts about other people but never acted on them in any way.

The truth is, she never would have found out about any of those things if I hadn't told her. Nobody else knew, and there was no way for her to discover them. But I didn't feel right keeping them to myself. It felt dishonest, and I felt like she deserved to know everything. I wanted our relationship to be built on complete honesty, even if telling her meant facing my own guilt.

Yet I still feel like I've somehow betrayed her, even though I never actually crossed any boundaries. The guilt is eating me alive. I keep replaying those weeks in my head, wondering if simply having those thoughts or changing my wallpaper makes me a bad boyfriend.

I'm only 20, and I've almost never been someone who cries. But over the past few months, I've cried more than I have in the rest of my life combined. I constantly feel anxious and guilty, and I can't seem to find any peace, even though I know I never actually cheated or crossed any physical or emotional boundaries.

One thing that's probably going to sound irrational or even stupid, but it's honestly the biggest source of my anxiety, is this:

I truly believe my girlfriend is the person I want to spend the rest of my life with. I love her more than anyone, and I can't imagine a future without her.

My fear isn't really that she'll leave me because she already knows everything. My fear is that, because of those weeks where I had doubts, changed my wallpaper, referred to her awkwardly, and had thoughts I never acted on, I've somehow ruined my chance to spend eternity with her. I believe there's something after this life, and I'm terrified that I don't deserve to be with her there because of what was going through my mind during that period.

I know that probably sounds irrational, but those thoughts have been consuming me.
She said she's tired of seeing me cry all the time because she can see that I'm not okay. She told me I can't keep punishing myself for something that's already in the past, and that if we're going to move forward together, I have to stop carrying this guilt everywhere I go.

The thing is... I want to believe her. I really do. But no matter how much she reassures me, I still can't seem to forgive myself.