What To Practice?

Hello guys, I'm Mauro from Italy. I'm 18 and I play guitar since 2006. I'll get straight to the problem: lately I don't know what to practice. I think my level is around intermediate/advanced, and I'm probably on that period where most of the teenagers are into that kind of shred stuff like Vai, Satriani, Petrucci, Malmsteen and all those well-known virtuosos. Anyways there are so many lessons to choose from and I really have no idea about what to practice to improve my technique/theory and all that stuff. I have tried some of Muris stuff, but I gave up because I tought it was just too difficult for me. I have also tried a lot of other lessons, but most of the time I gave up after reaching half-lesson... sometimes because I tought it just was too hard, sometimes because I had a lack of motivation and so on... I feel like I just can't stick to anything. Do you have any tips? Thanks a lot.