I'm surprised you found Bonaire to be so expensive.
I've been there 3 times and found it to be reasonably priced if you stay at the less expensive places, cook our own food, and work out some sort of "long term" arrangement with the windsurfing cocessionaires.
You would need to rent a car, or perhaps a motorbike, to get from shopping to where your staying and out to Lac Bay each day.
I think it would be pretty much a waste to spend your 2 months "learn to windsurf better" vacation in Florida in the summer time. Too hot, very little wind, from what I've heard.
Why not go back to the Outer Banks of NC.
More wind, you could tent camp or bring a nice camping trailer. Renting a house or motel room would cost more than Bonaire I think, as summer is the "high season" on the OBX.
Your current skill level could be the result of:
Not the right gear for you entry level skills.
You didn't learn to windsurf from instructors who teach "the easy way".
You haven't had the right kind of gear in the right conditions to really progress
Some other reasons, mostly having to do with gear selection, sailing conditions,
lack of instruction.
Basically the "secret" to progress, in nearly all cases, is some determination, and seeking out the right kind of instruction and gear to help you progress quickly.
Techniques and boards/rigs that you (or those who have advised/instructed you) think are "kiddie pool" or "training wheels level, may be exactly what you need to progress through the basic skills levels and launch you onto bigger rigs and faster boards.
Struggling with older heavy gear is one of the main things that holds people back.
Trying to "teach yourself" without good basic instruction stops many from ever really
getting going in this sport.
What gear do you have, and what lessons have you taken?
Identifying the gear you are trying to learn on may identify why you aren't making much progress.
Is the gear you have suitable for the conditions where you sail?
Have you "jumped" onto gear that too advanced for the skill you have?
Did you miss some of the basics by skipping to more advanced gear a little too quickly?
Are you trying to learn with gear that's simply too big and too heavy for your skilll level?
Thesa are all valid and pertinent questions.
Hope this helps,