Sounds like you really do love her. If you have already kissed as you've said then i think she'd be understanding if she were to be overwhelmed by your letter. I would go ahead with it if i was feeling that strongly for someone who is single. The question you need to ask yourself is would you be ok with just being friends with her?
I don't see the age difference as a problem. I've dated a woman 17 years older than me. Age is just a number. We've lived many many life times before this. Love is all that matters.