Whoever diagnosed him with the TBI should be able to answer some of your questions. It may be writing things down, getting him to repeat things to you when you ask him. But overall, you shouldn't have to "manage" either him or yourself when he lets you down. I can imagine it's exhausting!!!! I have a TBI and short-term memory loss, and I rely on my husband's memory. He often writes things down for me. But, see, I accept that I have a TBI and have had it 11 years. I've had time to find out what works and what doesn't. Even then, both of us will slip up and there's my dentist calling to see where I am. I just accept it. Maybe he needs re-evaluation AND TBI-specific counseling.
If his bipolar depression is "severe," then I would conclude that he's not on the right medication. He really needs to go back to his therapist and get re-evaluated. It may be that adding the right medication, or changing his meds entirely is in order. Most of the people I know with bipolar are pretty well adjusted after a trial and error period, in which he and the dr. would be seeking a safe and effective drug. Are you sure he's taking his meds?
I hate to say this, but if he isn't willing to look for a change for the better, you'll have to leave him for your own sake. As it is, your relationship sounds a little co-dependent. Make HIM make these changes, and sit there with him to be sure he makes the appropriate calls. Make sure he understands, even if you have to write it in a letter, that you expect him to give 100% to your relationship, and that includes taking care of himself. If he's not willing, I would walk. I would also get counseling for myself!! I know that's hard, and you've invested a lot in this relationship, but you didn't sign up to be an adult babysitter.
The best of luck to you!!!!