Mental stress is never good, even for healthy brains. The science shows that multitasking is detrimental to even healthy brains.

I doubt you have less intelligence. More likely you just need to learn new ways to access that intelligence. Many intelligent people have never had to focus hard to complete a task so they have never learn what it takes to focus. By giving yourself a chance to focus, you will be able to access your intelligence.

A key part of any brain injury or stress is quite simple. As we do tasks over and over, our brain learns shortcuts. The square of 4 is 16. Simple short cut. The long way would be to stop and think, The square of 4 is the same as 4 x 4. What is 4 x 4 ? If you give yourself a break and slow down to think, you may over time progress through this as:

First try,
What is the square of 4 ?
The square of any number is that number times itself.
The square of 4 is 4 times 4.
4 times 4 is 16.

A later try,
What is the square of 4 ?

The square of 4 is 4 times 4.
4 times 4 is 16.
The square of 4 is 16.

After even more repetitions,
What is the square of 4 ?

4 times 4 is 16.

Finally,
What is the square of 4 ?

16.

This same process happens with developing muscle memory (typing), walking up stairs (first you look at the stairs then later you don't need to)
driving
and many thought processes.

In chess, the way each individual piece can move is memorized (diagonal, straight one space, straight until a piece is encountered, 2 forward 1 to the side, etc.) But a skilled chess player never even thinks about how a piece can move. It just becomes intrinsic to the piece.

So, give yourself a break and time to relearn the many shortcuts and you will start improving. Getting frustrated will only make things worse.

I have lost and regained these skills multiple times. My personal challenge was mental math and trying to beat the cash register in the days before bar codes when items had price tags. I'd suffer another concussion and lose this skill then practice and get it back.

The tinnitus may never go away or may come and go. The only solution is to learn to ignore it. There is no treatment.

Stress makes cognitive effort like trying to untangle a string. The more you pull, the tighter the tangles. But, shake the string loose and the tangles will be much easier to untangle.

Short term memory is often a result of focus. Example. Learning somebody's name. A common problem at a new introduction is simple. We are distracted by thinking of our own name that we want to tell the other person so we do not fully hear their name. If we stop thinking of our name and focus on hearing their name, we will do much better.

Is this making sense ?