I cried reading your post. I know the pain. It's like mourning your own death - but somehow having to go on living.
Please try not to give up. Your posts are ordered, coherent and thoughtful. You are NOT without ability. I read your words, and know that you will improve. You are WAY far ahead of where I was at your stage. The neurologist told my husband my case was hopeless.
For now, I'd say, give yourself a little time, and do kind things for yourself. Then, start to look into cognitive therapy. Attention Processing Training transformed my post TBI world. When I started, I couldn't count backwards or even circle all the triangles on a page of simple shapes.
Your passion for teaching is undiminished. I feel that you will improve and predict that you will find a niche helping other TBI patients to do the same.
The brain is amazing - and so are you.