Assuming she has every STD in the world - mutual masturbation is still no risk. I'm including some links that will help explain why.
If you had rubbed your penis against her vagina and butt, well, that depends on how long, etc. In theory, that could put you at risk for herpes, syphilis, and HPV - all those are spread by genital skin-to-skin contact. However, they require some friction, so it takes more than just some brief touching.
I'm not sure what's happening with your sexual activity that you've already had 2 STDs and transmitted them to your current gf - maybe you need to start using condoms, or ease up on the pot when you are out with friends, or something. You would know far more than I would. (I'm not judging you at all - just trying to help reduce your risk.) Just something to think about.
The scar wouldn't increase your risk so long as everything is healed up.
Here are some posts from our experts (when we had them) that may help -
- read this thread thoroughly. There is an excellent explanation about why certain activities are not risks, and why others are.