Got Hit Really Badly, While In Shock And Denial Agreed To Tiny Settlement, What Can I Do Now?

I got rear ended really badly a couple days ago. They are fully at fault. I was fully stopped at a red light and they were likely speeding because of how strong the hit was.

Car is intact, but has tiny holes from impact.

I was in shock and denial and panic attack mode for two days and made really dumb and self detremental decisions. I'm going to the ER to check for damage now that I've come to my senses.

From good I took pictures where you can see both cars from the side on the street and a couple other ones. I have their insurance. They don't have mine. However, over text hours after leaving the scene, I agreed to a tiny sum (under $300). They did not send it as agreed initially but did send it later. I sent a confirmation of recieval.

Do these texts hold any legal power? Can I file a claim and return the money?

I have a call with a lawyer in a couple days but I don't want to wait with claims since the sooner the better. What are my chances? Does it hurt to file claim and see what happens?

Also I already know I'm an idiot, please be constructive about steps forward.