I screwed up

I screwed up big time. I've been a horrible girlfriend to my amazing hardworking boyfriend for a few months and he is sick of it and almost broke up with me but he is giving me another chance but I just feel like crap I don't know what happened this is not me I'm never like this can someone give me some advice on how I could show him how sorry I am or anything to help I don't want to lose him he's my best friend and my everything and it's all my fault