She probably didn’t see you in her future anymore. People’s perspective change all the time; maybe she wants to get married, explore other options, etc. You guys have been dating for most of your college years, and she may feel she has missed out on knowing the person she is as a young adult outside of a relationship. Wish you the best buddy.
Crazy that because “it was sudden”, she had to have cheated. Breakups are rarely sudden from both perspectives. At least from my experience, I know that I’ve never “suddenly” broken up with someone. I’ve thought it through for a long while and considered everything. If you don’t see what’s going on, that sounds like a you problem.