My girlfriend admitted to me that about 2 years ago, she kissed and touched a girl. I'm really sorry if I offend anybody but I have homophobia. I'm having some mixed feelings about this so please help. Thank you

How long have you been dating?

I think you understand that this is about you and your thoughts and opinions. Her past is something that she cannot change and is part of what makes her who she is. If you cannot let her past go and accept her as the person she is today, then you need to let her go.

You say you have mixed feelings. Are they chasing each other around in your head making you confused and dizzy? Try writing them down as a way to focus your thoughts and probably your imagination.

Why do you have a problem with her past? Is it general thoughts and opinions causing issues or specific to her? Is it because of the thought she might have been attracted to another female in the past? Are you afraid she is still attracted to females or that she might leave you for someone else?

It is fairly common for many young girls to experiment. I would say, why do you feel this way ? This is more your issue , and it appears you do not love her enough to forgive or over look

Want to start with your homophobia here?
How would you define it, what is your fear, your dislike, what do you think 'happens' when someone realizes he or she is gay, or who experiments with sex?
Has it ever crossed your mind, and scared you to death?
Men especially can freak out because they might get an erection around a gay guy, or even a straight one.
Women might think something like 'another woman might understand how I feel, what I like.' They might feel turned on by something attractive in another woman, even if they are straight.
There are countless variations on the above. It's a sliding scale...
What about you, your life, your childhood, etc, led to fear and dislike?

Work on dealing with your own fears and issues, and don't put them on others. Not very healthy of any partner.