What to do in this relationship?

My gf cheated on me 8 months ago, i took her back because I love her. Yesterday she told me the real reason why, and it was because she knew I would be there even after. She has been perfect the past 8 months though. She feels bad that she took me for granted. Yesterday she told me this and gave me a choice. To leave her or to give her a real chance at this relationship. ( for the past 8 months I been kinda of a a$s to her. any good tips? btw the reason she took me for granted is because I'm not the best looking guy on the planet but not the worst. I do have girls over me when I get into shape and actually try. But at the moment I am kinda a little chubby.. But anywhoo any advice will help