I had a similar injury like yours and had concussion and brain contusion (2.4 years ago). I was progressing for the first 4-5 weeks then everything got worse just like that.

Dizziness, imbalance, nausea, fatigue, tinnitus, smell and taste disorder etc., But very slowly some of the symptoms has improved like smell and taste, fatigue but dizziness, imbalance, nausea still doesn't want to leave.

I have some Sensory Processing Disorder and think it is mostly coming from ear and vision. I went to all kinds of specialists Neurologists, Neuro-otologists etc., many MRIs, CT Scans, ENT tests etc., No one was able to help me so far. They all tried but nothing worked.

Few things helped but all of them are temporary:
1. Xanax (not on it anymore since it stopped working and don't want to go up on dosage)
2. Trigger point therapy/Myofascial pain release on the neck close to the skull
3. Neurofeedback

Now I am going for Vision Therapy once a week and it also helps only temporarily.

I work from home and still long ways to go to a office set up and handle all that.

I can feel what you are going through with that dizziness/imbalance etc., and is a tough one to diagnose and treat. I am going to see my ENT ask about Perilymph Fistula which can cause lot of terrible symptoms. No MRI/ tests can show that and the only way is exploratory surgery and is a hit or miss.