It seems to me that you are very anxious about this. Have you tried to reduce your anxiety? That might be very helpful.

So, your words have been slurring since you were in 6th grade? And it happened spontaneously? Did you play sports?

Do you have any other symptoms? Or any new symptoms?

It might not have anything to do with the forceps, or the bump to your head i the 3rd grade, it might just be random genetics/developmental issues. Regardless, speech therapy might help you.

Instead of asking your Dr. for an MRI, have you just explained to them what symptoms you're having that is distressing you to see if anything can be done to help you cope with them? Maybe they will refer you to a good speech therapist?

I think seeing a behavioral therapist might be able to help you with to reduce the anxiety.