Does your girlfriend know that you've never fully explained her side of the story for your fight scenario to your friends? And you've never explained to her fully why they won't warm up to her?

Dude. Your friends and your girlfriend don't get along because YOU have driven a wedge between them. You just drift along after dumping your misunderstandings on them and just because you have it all figured out, you don't bother to give either side the full story. Have you vented to your friends about other problems you've had in your relationship? Have you ever told them anything good about your girlfriend?

Seriously, if you want this to get any better you need to open up the full story of everything that's happened to both of them. Tell your girlfriend why your friends don't like her. Tell your friends why it was your fault that they misunderstand her. It has to come out of your mouth. I get that it's been a long time. But this either has to be cleared up now, or you will lose one or the other. Honestly if I was your girlfriend I'd completely rethink the idea of marrying you if you were this bad at having her back.