Hi Guys

I've been trying to get my playing up to speed, I'm hovering around 120BPM straight 16th, and I have run into a problem.
I can't comfortable play the low E-string with my Plucking hand.

My forearm rests fully on the Bass. (T-Rex claw)
MY thumb is jumping between anchors on different strings.
My fretting hand is pretty relaxed with the thumb behind the neck.

Now I know that I should lift my arm of the bass, but every time I do so my fretting hand pushes the neck back.
To stabilize it I would have to use my thumb, which causes pain in it.

So I'm stuck between resting my arm on it, which means not being able to play the Low E comfortably or lifting my arm up,
which means to limit my fretting hands mobility.

It also doesn't matter if I'm standing or sitting it's always the same.
Strap height seems to not fix it either.

I've tried finding a solution but to no avail. So if one of you could explain to me what I'm doing wrong or
point me in the direction of a instructional YouTube Video that would be much obliged

Raise the neck (don't play horizontally).
Don't squash your thumb flat -- keep it rounded (fretting hand). If you squash it flat (or have it nearly parallel to the neck) you're going to develop some pain in the wrist and set yourself up for arthritis (or you may already have it).