How do you fix a really bad relationship?

Me and my girlfriend have had a lot of arguments since New Year's Eve. Most of them because of what she did or didn't do. In the end because I love her so much I've apologised about my approach to the matter. Our relationship is in a really bad place right now. And I just want to fix it again for ONE LAST TIME, yep just this once and if it don't work after that I'll call it quits.

I've confronted her about the guys she keeps giving her number to and some she actually saves them with girl names. We've argued about it and she said she won't do that again and she wants me not to look in her phone.

Apart from what has happened I just want me & her to be good again. In all honesty I wouldn't stand for this type of **** but because I truly love her and I've known her for 4 years and been together for 3 years.

I know some might say leave her and ****. But the way I feel for this girl is like nothing I've ever felt. So if you can help me with advice on what to do to fix it that would be great.

Thank you in advance.