if you can't work 8hours a day, just don't. A lot of professionnals don't go over 5hours, others 4... It all depends on concentration and on your working method (you can work 8hours with shity work and make little progress or working 4hours with producive work and make huge progress)
Sometimes you also need to do a break, don't work for ~~ 2 or 3days, try to relax , avoid feeling guilty, and come back to work when you "rested" a little.
Also you might want to think the work differently, more like a discovery of the piece, an animal you have to conquer, a chess game, others... It can help
if you just work work for result you can be bored quickly because the result doesn't always come too fast and you loose your will and courage, working an instrument needs patience!
idk exactly how u work etc and I dont think i'll be much of a "technical help" since i'm not guitarist
but i this post might help u.