If you don’t tell her, you’ll regret it forever. (No pressure.) Especially if, years on down the road, you discover that she held similar feelings for you.
Sometimes folks do fall in love with their best friends (and we should all be so lucky as to have our best friend as our partner!). That is nothing to be afraid of.
Ask her if she has a free hour coming up, sit her down … (breathe) … and tell her how you feel. Be prepared that she may not feel the same way about you. But also enjoy the release as you get these feelings off your chest.