Whilst I agree with @LucyPurrs that putting your thoughts in writing may be easier for you, I am a little concerned that it may also potentially push her further away. If I had a male friend who declared himself in writing to me, I would feel cornered unless it was what I was hoping for. If I wanted to keep the friendship but didn't (or couldn't for reasons of my own) feel I wanted it to become a romance I would distance myself further.
If I was hoping for a romance, I would definitely be encouraged by a Valentine's invitation, and see that as the perfect opportunity to deepen the relationship. The fact that she responded with a curt 'no thanks' doesn't bode well IMO.
Sorry, I know that isn't what you want to hear!
If you want to keep the friendship and are prepared to accept it may never be more, I would be giving her a little space right now.
Hope it works out well whatever you decide.
I know, but I don't want to know...