I usually hold grudges whenever my girlfriend and I fight
Why do you hold grudges?
I think that holding onto a grudge can be helpful in life.  You can just bring it up whenever you are bothered.
You shouldn't hold onto grudges, because this doesn't solve any problems.  If you have an issue with your partner, let her know how you feel, and you can both try to fix the problem.  You may want to see a couples therapist if the issues are too deep to fix on your own.
Are you a therapist who can help with this?
I'm not a therapist, I'm just here to answer questions and to give advice.  I would recommend speaking with a professional therapist, but in the meantime, try not to hold a grudge against your girlfriend.  If she has apologized for something, you should be able to forgive her for it.