First of all you have to decide what is your purpose for divulging the secret now. Is it for your benefit or theirs?
If it's something you did a long time ago and no longer are doing then odds are you simply want to relieve yourself of (guilt). Sometimes carrying guilt is the price we pay for our past actions. It makes no sense to reveal it now.
Having said that I see you have this question under (crushes). If you have a "platonic friend" whom you have had a crush on for years it's probably safe to assume they have no interest in you romantically. Generally speaking over the course of time there would have been some flirtatious moments or sexual innuendo that would have provided an opportunity to share a kiss or propose going out on a date. Platonic friends are like siblings with different parents.
If there has been no indication that they are attracted to you in any romantic sense it probably will come as shock to them for you to suddenly reveal you want to date them. If you want to maintain a close friendship you'd be better off letting that dream go and find someone else to love.
On the other hand if you believe they are "the one" for you and you're willing to end the friendship if there is no mutual interest then by all means sit them down and tell them how you feel. Ideally you'd do this when they are not in a relationship with anyone else.