Dear Guest, you have my sympathy. I know what it's like to be sick of the song by the time I've learned it. What I did was to lock myself down in a cellar for three months and learned the songs that I sang off by heart. I must have sung Folsom Prison a million times. However it took less and less time to learn each new one and it wasn't too big a problem after a year. Learning songs off by heart is just like learning the guitar; it gets easier after you've done the countless hours of work.
Now about strings and fingers. I think that you have got it wrong. An expensive guitar is not necessarily harder to play than a cheapie. The same applies to mandolins, I guess. If you do a search among the old threads, you'll get loads of advice on this. If you don't know how, just ask.