"I've read on several threads that objects/ humans need to have a certain mass to cause a concussion. Like a small child for example cannot cause a concussion because it doesn't have enough body mass.

Does that mean that any object which doesn't have enough mass ( I don't know say under 5 pounds) cannot cause a concussion no matter how hard I hit my head with it? "

The first comment has very little basis. Mass is not the key factor. G force is. The formula is such that something of small mass with enough velocity can impart the minimum force to cause a concussion. Or, something of large mass with less velocity can cause a concussion. There are other variables that also come into play.

But, I don't think you will be able to live a life of calculating every contact with your head. Hits happen. The brain tolerates most of them. Anxiety can make an impact that did not cause a concussion appear to be a concussion. We can train our brain to react to different stimuli. Our brain will react as if a contact is a concussion if we think it was a concussive force or for some, we just think it may be a concussive force.

I think it is better to just ignore incidental head contacts. Avoid risks and try to tolerate the accidental impacts. Laying your head down on a mattress or pillow is not a risk. Touching the headboard is not a risk in most situations.

I think your anxiety needs to be resolved away from the concept of concussions. Your anxiety about concussions is likely only a small part of your anxiety. The anxiety you have about the myriad other issues in life is just as important at resolving. Have you sought counseling to help you learn thought processes so you can let go of your anxieties ?

Rather than trying to think "This was not enough force to be a concussion," You need to just think "I'm OK." and move on.