sounds like you could just use some lovin'
ain't no shame in that.
whether you tell her or not is obviously something only you should decide. but here's a couple things to think about, other perspectives you might want to take into consideration.
on one hand, it would relieve the burden off your shoulders of carryng this unproclaimed adoration (notice i didn't say love).
from her perspective though, what can she do about it? now she's in a relationship, with someone else proclaiming their deep feelings for her. what exactly do you expect her to do?
i think we all have the hollywood fantasy of proclaiming our eternal love to someone and having them drop everything, running towards arms outstretched at us and things will be happily ever after. from my experience, life isn't so optimistic. heh, that just suddenly reminded me of a line i heard somewhere. "Shit in one hand, wish in the other. See which one feels more real."
ask yourself how deep are these feelings really? could it be a crush, or is this something that dramatically influences the dynamics of your relationship?