"i am 18, i have been dating my current gf for three years... i am in love with her and care for her as much as anyone in the world... the last thing i would ever want to do is hurt her... i have had a "crush" on another girl for a while now (year or so) and now have found out that she has feelings for me also... now, i dont necessarily want to date my "crush", but i feel held back in my current relationship, and with her going away to college in 2 years, and i in another year, i just dont know..."
If you don't see anything happening with the crush, (and I do mean ANYTHING, even as slight as a peck on the lips) then I personally wouldn't say a thing to your GF.. No point in hurting her feelings for something that was never acted on. Once feelings become action is where things go bad.
I'm a very happily married guy, and I love my wife more than anything or anyone including myself... I'd give or do anything for her...
...and at times, I am painfully aware of how many amazing, funny, smart and beautiful women there are in the world - a few are very intriguing to say the least... But nothing can replace the connection and happiness I have with my wife, so I force myself to never act (in any way) towards the few evil thoughts that might pop into my mind from time to time... In such a case, you don't wanna say - "hey, I love you babe - but that girl Barbara at the gym is really amazing, she's so smart and pretty, I really like her and we have such a great rapport".... That'd be horrible, if you're not gonna act on it, swallow it deep down inside and focus back on your life..
If you're certain you're gonna act on it, tell her BEFORE anything happens... She won't be thrilled (or anything even close) BUT, at LEAST you spare her the pain, humiliation, and rage that she'd rightfully have by cheating.
my 2 cents at least.