Because you're putting a crazy amount of pressure on yourself.
A hare won't run if you put a millstone on it, no matter how much it was built to run.

Inspired, suggested:

The thing you need to survive, emotionally
Or that you currently think you do
Making art
in order to retain self-worth
May be so important that you are paralysed
Fear of failure
The motion to even try to draw hurts
strain in the bones and deep muscle
whole body is tired and wants to sleep
held in perpetual state of restlessness

I see scientists on PBS
And I think, when I was a kid they inspired me
Now here I am, did not finish college
See them and there is a tinge of sadness
and that tiredness
I am not with who I need to be with
The other scientists.
To go back to college
Such strain
Want to lay down
First need to fix other problems
Heavy burden
May be all emotional? but that is still a burden
What other kind of burden is there? If the emotions were not affected, no ordeal would really be a burden?

Fear of Doom
That the next action if resulting in failure
may lead to a slow miserable life up until death
That the next action may lead to an emotional breakdown
Throwing the self into panic
And if there is no one there to help
Panic, for how long?

You are already in a state of emotional turmoil
the burden gets heavier when it is perceived, that quality of life depends on the outcome of a very specific action.
That in carrying out this action, there might be failure
That happiness may be gone forever more
That the self might revert into an earlier state of misery
A throwback to suicidal times

But life is complex.
There are many ways to survive.
The Self thinks: But am /I/ strong enough!?
I think -- if I had to do certain things
As good as a slave, sucking all joy from me
And in the things I want to do,
Would not be able to pursue them for the energy already drained.
And the things i want to do, I have trouble doing.
Even writing, the easiest of these, can be a struggle at times.
I wonder -- will I ever get myself together?
I think I will.

Circumstance
In an incompatible environment, the burden is heavier
Surrounded by angry people, or fools, or abusers
Their voices get in you and attack from inside
Fear of outside consequence
Surrounding factors make it hard to resist
Hard to work
Will there ever be a way out?

I sometimes see The Good
Its presence, and the miracles I see
Keep me going in the strain
Use it all, the struggle
Understand your own
Study others as you find them
If suffering is prominent to you
Unique stories of pain, shared for empathy.
To better understand and liberate the self.
And to aide the greater human network
Suffering transmuted into connection
Light from the Dark

No one's worth hinges on a single aspect of their identity or skill. You are worth more than what you create. You are not SIMPLY your art; how can a creation be more valuable than its creator?

I know how it is when you cannot seem to create, and it feels like everything is closing in around you, but the problem is we, as artists, in these situations, wind up perpetuating the very stress that keeps us from our craft in the first place.

You, as a person, have inherent value, apart from any interests you may have, hobbies you may pursue, or things you may create. Your impact on this world far eclipses the tangible 'things' you produce, and we would perhaps all do well to remember that about ourselves.
Relax. Take time to enjoy yourself. Take time for you. You deserve no less. Your existence and value do not hinge on your ability to provide artwork (beautiful though your work is,) to placate the masses. You're worth too much to tear yourself apart over so little.

I've felt this far too often myself, especially since my spouse works at what my brain continues to insist is a "real" job and I'm too socially anxious to even set foot out of the apartment, let alone get one myself. So I do art. But I have to do art, which makes me not want to do it, because suddenly there's all this ungodly pressure.

I don't know what to tell you on how to fix it. All that I do, personally, is try to take it one step at a time, one small part of the picture at a time. Something like "I am working on the hand now." and then draw the hand. And that's all. Don't think about the other parts as best you can, just focus on that hand. Or whatever.

Whatever else happens, I hope you feel better soon.

I can deeply sympathize. Sometimes all that one can do is put down the pencil for a while and think about other things.
Or sometimes working on other art forms can help. Actually, free writing can be pretty helpful (well, at least I have found it helpful) to just start letting it flow. Just writing down anything that comes into your head and to start forming something from that sort of activates different parts of the brain.

But most importantly,
You, more than anyone else in the universe, deserve your love and affection. You will never be worthless.