How does one free oneself from being trapped in one's imagination?

Try to see the light at the end of the tunnel. Pull the trigger.

You take the red pill.

Ask who am I who is trapped.

Go outside and take a walk. Make it a regular habit.

If you can’t take a walk, just step outside and breathe in the fresh air.

Use it.
Write stories or just a Journal of how you feel?
Don’t professional writers spend a huge amount of their day in “imagination”...that is how they are inspired to write such great novels!
Perhaps you may have a Great Novel just waiting to be written?

Seconded- go out and take a walk. If you can, chat with someone, even if it’s just the cashier at the store or one of your neighbors.

Third it – get out of your room and you will get out of your head.