I live in Florida and established child support from my then soon-to-be ex-husband for my daughter when she was about 4 months old. I didn't ask for "back child support", nor child care coverage, nor health coverage, just the basics because I was somewhat afraid of any drama/violence that would follow from the child support being "too high". Of course I greatly regret this now and need more than the $243 that is supposed to come every month. My ex doesn't even pay regularly, and seems like he's going to continue to wait to the end of the year for his tax return to be taken, which is hard on me. My daughter is almost 18 months now and needs to be in daycare so that I can work and go back to school this fall full-time. And my mom has to get a 2nd job in the day to help meet our expenses, so she can't volunteer her time to babysit anymore. I was told that she must already be in daycare for me to request an increase in child support, but I can't afford $550/month. What can I do? Also, can I get compensation for the child care that has been provided by my family all this time? Thank you.

Who told you that your child has to be in daycare first? Have you actually applied? Granted, it's only been 14 months, so they may say that's too soon. There may be an arbitrary aspect to it based on who you talk to. This is all at taxpayer expense, and the taxes aren't exactly rolling in to pay for the employees you are talking to. Try someone else. I assume that you can't afford a lawyer, who would be your best option. Even if it means postponing school.
You can't have the past reconfigured unless you can prove that he deceived the court.
I highly doubt that you will get money for family babysitting as daycare, even for the future.
When you get a job, of course, he can apply again too, and ask for a reduction.