First things first.
Remove your starter, take it to a small engine, or electric motor/generator repair shop and see if they can test it, to see if it produces adequate power. It has to be able to spin your engine fast enough to start it. If you call ahead to a few places and tell them what you are doing, then they can tell you where to go.
Well, that was a bad choice of words, now wasn't it?
They can tell you where to take starter motor to have it tested.
One word. I have replaced starters with, off the shelf, "brand new" _and_ re-manufactured, that were bad, out of the box.
'Cause if the starter is toast then it ain't gonna fix it'self.
If you own a volt meter and are not afraid to tread where few venture, then let me know and I will give you some direction, starting with a logical explanation of what we hope to determine with each check. "Be patient Luke"(Obi-Wan Kenobi)