Hello guys! I have a very specific guitar technique problem. I've been lucky enough to get some lessons from an extremely skilled guitarist, who always keeps his thumb behind the neck. Now I love the approach, since it does give a good ground to build consistency on - I mean, always behind the neck, that's one less problem to worry about, right? My problem really arises when playing lead patterns in the higher register, especially stretchy, chordy kind of things. Patterns, where I have to hold two fingers down simultaneously in the upper frets. A really tricky one is one where I have to slide from below, and hold the 17th fret in the G string and 21th fret in the D string simultaneously. The thumb just feels really uncomfortable and I find it hard to have any kind of strength or dexterity there. The thumb just won't bend to be in the place I feel it should be. Definitely it's not straight behind the neck, like in the lower register. Now this is doable on a sitting position, when you can hold the guitar far on the left knee, but when standing up, things get challenging. What kinds of tips would you give me regarding the thumb position in the upper fret register? Really thankful for any tips. I feel that this is certainly a key roadblock for me at this moment, holding me back.