How do I get resentment of my ex-crush?

I have liked this guy ("Alex") for over a year now. He knew I liked him,and he was really nice about it-we would still talk as friends,etc. The year before,my friend had asked him out (I didn't like him then) and he said he "Didn't want to go out til he was a certain age"(which was far into the future. Then today,he asked a girl ("Katie") out. Katie is very nice,and I'm happy for her,but I feel like I wasn't good enough for Alex. I'm upset,and annoyed that Alex made a lame excuse to my friend. Plus I can't avoid being around Alex and Katie,and I feel really uncomfortable (and rejected.) Please help!:(