I know my girlfriend slept with an older man but it doesn't feel as bad as if it was someone my age
She still cheated on you, so why does the age matter?
I don't know why but it's hard for me to accept when an old guy hits on a girl I date
I'm sorry your girlfriend cheated on you, so why are you still with her? There is no way to justify such things like you are doing. But if you want to forgive her and work things out, that is your choice.
I feel like I am in love with her so I want to continue to be with her.
She should have more respect for you if she loves you. You shouldn't cheat on your partner with anyone and you shouldn't be with someone that cheats on you. Maybe you will be able to move past it if you are able to communicate effectively.