BBV -- first off, go here:
www.marriagebuilders.com and read the basic concepts section.
The answer is: if you still love the girl you are with, you should not do anything with the other girl. Right now, you'd just be doing it because you're pissed, not because you want to move on. And say things did go south afterwards, would you always wonder if it was because of your actions? (even if it wasn't)...
There is plenty of time -- you don't have to make a decision right now. First off, you need to work at getting back in touch with yourself -- do some things for you right now, regain your self-confidence. If things don't work out with your girl, take some drying time -- say a year -- just to be by yourself, remember, YOU are responsible for your hapiness, depending on others to provide it for you is a very unsatisfying thing.
I guess my point is, is that if you really want to work things out, fucking around will not help. Likely, you'll end up in love with 2 girls and be confused as hell. Wait until you no longer love the girl you are with, where her leaving would be a blessing, not a traumatic thing -- then you can consider other options.
Here's a few books:
Love Must Be Tough -- James Dobson
Give And Take -- William F. Harley Jr.
Relationship Rescue -- Phil McGraw
Also, do check out the marriagebuilders site -- there is a TON of great info over there and an excellent chat board. If you get serious about it, I strongly recommend the MarriageBuilders weekend -- my wife and I went and I don't know if we'd still be together today if we hadn't --- no shit.
Good luck man. E-mail if you want. Later.