Maybe this is how we know what it is, arwen. Maybe when it doesn't fade, it's true.
I have to agree. Having had two very serious relationships, I see two very different results out of them. I thought both were love at the time, but one lives on...very different than the other.
I can say without a doubt that I truly loved my first girlfriend. I still love her to this day. It doesn't interfere with other relationships; God has given me so much of His own love to share that it doesn't reduce the love I can give to anyone. At least I hope that any girl that comes into my life in the future feels that way.
In the end, time is the key. Feelings, infatuations, are fleeting. The only way I've seen to truly know if it is true love is by consulting God and giving it time. In watching a few of my friends repeatedly crash and burn in relationships, one thing common between most/all is how quickly they throw themself "all in". But that's a whole different topic.