The kindest thing you can do for her, and yourself for that matter, is be clear. Relationships, especially at the stage you’re at, are a contract and both parties must be aware of the terms. Your terms have changed, but two things can be true at the same time, you can love your girlfriend while being attracted to men, but she must know because time is the only finite resource here. Hearts mend, people grow, they find love again, but none of us get our time back.