Help with trills/semiquavers

[reginayanster]

Hi. So. I've been playing Debussy's Pour le Piano: Prelude
and I am having sooo incredibly much trouble with the
left hand trills/quavers near the middle of the piece where
it repeatedly plays D and E.
I've been using my thumb and first finger to do the trills/quavers
but they are seriously slow and uneven.
please help!!!
Thanks!

Oh oh and also in Mendelssohn's Rondo Capriccioso, near the end
of the piece, my hand cramps up during the long chromatic scale in
the right hand. Any way to help relax the tension and but still make
the right hand louder?

Again, thank you SOOO much for the help!