Try adjusting your strap to a comfortable height. Practice standing up. Could be factors.
Really if you could pick at that speed rate cleanly, accurately, and comfortably descending 16th notes, ascending should not be hard to practice and be comfy at all. Takes a lot of mastery to do those at that speed and konting practice na lang yan ascend.
What also helped me is doing lower note groupings kapag mahirap talaga. Instead of 16ths, I do 8th notes or one note per beat. If sixes are hard, I do threes. Then practice lang dun. It takes time pa rin to speed up but I get more control and comfort pa rin which is important for me.
Pero nanotice ko most problems sa picking can be solved through slowing down, having patience to work through it, and giving the right amount of time. Could take a long while to master some problem pero ayus lang yun own development mo naman yun.
I don't think there is any much reason as to why nahihirapan ka instead of the fact that you nee to practice it just some more.