There is always "belt dressing".

OMG!!!!
Please for the love of Pete, STOP putting things on the new belts! If you just replaced the belts and they are starting to squeal again then they have stretched. This is absolutely normal with a Miata.
Start by going to the auto parts store and picking up a copy of the Haynes Manual for your car. Then throw it in the bathroom for when you have some time to kill. READ IT and you will at least have an understanding of how your car works. You don't need to turn wrenches to understand when you are being taken advantage of.
It sounds to me that the belt is slipping and the battery cant stay charged. If the alternator (the thing that spins to make electricity for your car) cant keep up then you will have hard starting issues that you described. Checking the battery, the cables and the grounds are all good advice but you need to join the local Miata club and start hanging around folks that know these cars. Buying a pizza and a few adult beverages will save you a ton of shop labor......
Best of luck!