I *think* from what you said that your pinion gear is not extending as it
should to engage with the flywheel.  How hard is it to fix?  Well, if you
take apart the starter motor you might find the problem and be able to fix
it.  I did it once many years ago but I don't remember exactly what I did.
Otherwise, to replace the starter motor it's not very difficult.  The main
thing is to disconnect the battery before you start.  But, from what you
said, the problem may not be the starter motor.  If you were able to
jump-start it then it must be working, at least sometimes.  Was it, by any
chance, on an incline heading uphill when you jump-started it?  That might
engage the gear.  If you have never replaced the positive battery cable that
might also be the culprit.