I've had a similar experience to you when I was studying as well.
I was 26 at the time, we had been having a long distance relationship which lasted about 4-5 years. Every time we talk/chat/meet everything was all OK and I felt so in love. I was really emotionally attached to her, and I assumed that it was vice versa. Then one day, I called her cell phone and I think the phone was in her handbag and she must have accidentally pressed 'answer' only that she didn't know it. What I could hear was her talking to another guy while they were driving together. They were talking romantically.
I was furious and I called her sister to ask if she was having an affair. Her sister didn't deny, but she also didn't confirm. All she could say was "ask her yourself". In the end she admitted that she has been going out with someone else, but it didn't mean anything, blah, blah, blah. I couldn't stand the cheating so I broke off with her straight away. I was really depressed for a while as I really had thought she would be "the one" for me. But through being busy with work, life happened, I moved on.
I was told a long time ago that to find a good partner you need to have a match on: Spirituality, Physicality and Compatibility.
When I was young, I thought Physicality was the most important and I was really attracted to this girl. She was really attractive, but it was also her personality that I really liked. But something never sat right with me in terms of her integrity. If I ask where she'd been, sometimes she would just brush it off like it wasn't important. If I had given her some money to buy something, she would buy something else completely. Her promises weren't always fulfilled.
I've since become a follower of Christ, and it made me realize that the things that are most important to me all along are: Love, Loyalty, Integrity, Honesty, Monogamy. These things are so integral to my heart now that it's impossible to imagine being with someone who do not believe in these things, or who do not think these things are important. (Meaning, I would have regretted marrying that girl even if I had forgiven her, because she does not value these things).
I don't know if any of that can help you, but I do hope you can work out what's really important to you, and find the right course of action.
God Bless.