As far as this site or any therapy goes, it's important to express to your g/f that this is something YOU need. It's not shutting her out to find peace for yourself. That's something I had to go through with my b/f, also because of this site. He is not ready to come here and the fact that I do was very stressful for him. I didn't lie to him and say I wouldn't come here because I needed the feedback, desperately. I explained that I wasn't doing it for him, I was doing it for ME. It made me a healthier person to see what others went through and are going through. This site provides a level of understanding that can't be found in our every day lives.
I can understand a little of the jealousy she may feel, but if put to her correctly, that it's not a replacement for her, it's just another avenue you are exploring to help YOU, perhaps she will understand.
If you fall down 10 times, Stand up 11.