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"Why Can't I"
by Liz Phair, maybe? It was in the soundtrack for Thirteen Going On Thirty.
"With You" by Jessica Simpson
came to mind. Don't think it was in a movie though, but I remember it still playing the radio years after coming out.
I'm assuming this is something you heard quite a few times? You can check the Billboard year end charts for the years you think it might've come from. Just cut and paste to make a list, delete the stuff you can easily rule out, then listen to samples of the rest on Amazon MP 3. That's what I did when I couldn't remember the title or lyrics of this one cool song.
Not sure if I'd describe it as "peppy" or "upbeat", but I'm going to throw Kate Winslet's single offering, which I think might have been called "What If"
Straying from the "what if", but the above reply made me think of "How Do I Deal" by Jennifer Love Hewitt (released in 1999, featured on I Still Know What You Did Last Summer soundtrack).
"If it ain't broke, don't fix it. And even if it is broke, just ignore it and maybe it'll be sort of OK — like the environment."

A peppy, upbeat song from the 90's or early 2000's. It might've been in a movie soundtrack but I can't be sure. Female singer with a slightly high pitched voice. I think it was pretty popular and it still goes on the radio today (at least here it does). I cannot remember the lyrics but I think there was a "(what) if I" or something in the chorus. I'm sorry I cannot remember more.