Will my repentance be accepted for committing a forbidden act with my girlfriend, even if I don't break off my relationship with her, and does remembering the sin or its location affect the validity of repentance?

Your repentance is accepted if it fulfills its conditions: abstaining from the sin, regretting it, and resolving not to return to it. It is not a condition for repentance to sever your relationship with that friend, but it should be done if you fear returning to the sin. And if she is not among the righteous, then abandoning her companionship is more appropriate. You should advise her, explain the impermissibility of her action, and invite her to repent. Your passing by the place of the sin does not affect the validity of your repentance. Increase your seeking of forgiveness and performing good deeds that expiate sins.