I'm a Florida criminal defense attorney and I've handled many early termination of probation cases.
So you know, its not a letter you need but a motion. It would be titled Motion for Early Termination of Probation. While I don't have your specific facts and can't write the motion, I can provide a template for you.
If you have any pleadings from your case; the charging document, motions filed by your attorney, the discovery document provided by the state attorney; they will have the "style" or heading for the case....use that at the top of the motion.
The motion would state the following:
On whatever date, placed on probation for whatever offenses for 3 years. Conditions of probation included A, B and C.
As of this date, all terms and conditions of probation have been fully satisfied.
There have been no violations of probation or allegations of violations of probation.
At least 1/2 of the term of probation has been completed.
At this time, there is no legitimate need to keep the defendant on probation.
Obviously, you need to fill in the specific facts of your case. Keep in mind, as a person representing yourself, you do not need to be perfect, you just need to make it clear what you're trying to accomplish.
Once the motion is drafted, file it with the clerk of court and send copies to the state attorney and your probation officer. Next, contact the judge's assistant and request a hearing on your motion.
Assuming everything I stated above is accurate, it is quite likely the judge will grant the relief you're requesting.