I have a crush on a girl who is not my wife.

I am in a graduate school program and there is a girl in my year who I have developed a crush on. I just got married over the summer. I feel horrible. My wife is fantastic and I feel incredibly guilty for even thinking about another girl being pretty.

My wife and I have been together for more than 10 years. We have had very few relationship issues in our entire time together. She was my first everything. I was kind of a chubby and ugly kid. I had no experience with girls before university. I had a bit of a glow up in university and started to get into better shape. I got a bit more attention from girls, but never any that I was interested in before my wife.

When I started my graduate program I noticed a girl who I thought was really pretty. I never spoke to her. Midway through my degree we had a class together. Sometimes I would catch her looking at me. When we would make eye contact she would smile and usually I would smile back. We started to have conversations in passing but nothing in depth. I had another class with her and one day the only seat available was next to her. She struck up a conversation with me during a break and I mentioned I was engaged right before class resumed. She said "oh you're engaged..." sounding really disappointed and her shoulders slumped.

I don't have classes with her anymore but I still see her around sometimes. She always looks at me, smiles, and often waves. I used to reciprocate, but I feel really guilty that I have this crush. I have tried to stop looking at her or smiling when she does to me and try to limit our conversations. I feel horribly guilty for having a crush on another girl when I'm married, even more so because of limited interactions with this girl, and because I'm in my mid 20's and this seems so high school, but my crush never seems to go away.