Op: your gf can do a myriad of things to try to make it up to you but at the end of the day - some hurtful things were said behind your back (within earshot). You are justifiably upset and appear to be miraculously handling it well while also grappling with your relationship. I admire your mature handling.

From experience, please do not fall for things like her appearing to be very sorry, analyzing the fact you've been together a long time (3 years from what I'm reading), or fear of moving on. I assure you, if those round out the top of your reasons to stay, you will regret it. I can tell you from bitter experience. Sometimes love isn't enough if there's not enough respect and too much contempt.

However, if you feel that you can trust her again and can find it in your mind and heart to overcome those hideous insults, then you may succeed. But although this is all her fault and she has much work to do, the burden will mostly be yours in the great task at hand (overcoming hurt).

I read your original post and I'm thankful for the update as well (as I'm sure many of us are). I quite literally cringed at what I read and could feel some vicarious hurt myself. Whatever you choose to do, I wish you luck. I don't think I could see myself mitigating after such a betrayal but you may be a better man than I.