How can I remotely access my girlfriend’s cell phone without her knowing?
Go online. Download spyware. Install it on her phone. If you are in the United States, pray that she doesn't find out, or if she does, that she doesn't report it; doing this is a Federal offence that carries a sentence of up to 20 years in prison.
There's a harder but less risky thing you might try: Become a mature adult, realize that doing this makes you an abuser, seek therapy, and don't do it.