As you may recall, I'm a lawyer, not a doc, with strong desire to help. Found this:

What is the proper dosage for lithium?
Dosing for lithium varies according to your age, weight, and medical history. This medication should be taken with care, and only according to your doctor’s specific instructions.

Oral lithium comes in capsules, a liquid solution, and extended-release tablets.

It can take several weeks for lithium to start to take effect when using it to treat bipolar depression. A standard dose of oral lithium for an adult is 600–900 milligrams, taken two or three times per day.

To protect you from side effects and make sure you’re not getting too much medication, your doctor will draw blood to monitor your lithium levels.

https://www.healthline.com/health/can-lithium-help-treat-depression#dosage

She was diagnosed with bipolar disorder in the early 80s and was on lithium and other psychoactive medications for the rest of her life (she passed this past January). I have no idea what dosages she was on at any time, but I can confirm that she had blood tests to monitor the amount in her blood.

I need to find out more about the monitoring so that I can make sure I do it regularly.

I found that too. It was comforting.

results. I still suffer tremors from the antidepressants I took back then. Now it’s cognitive strategies and exercise. B12 helps me too.
I thought I had the depression under control. My current problems turned out to be the PTSD I’ve neglected. Working on that now.
Best of luck and keep moving.

Got a degree in clinical psychology. Gave me the tools to continue journey.

Psychiatric medications can be a lifesaver for some of us, but they can also be a crap shoot, because their use is still something of a guessing game, even after all the years they've been in use. There's no way to tell in advance how someone is going to react to a particular medication/dosage, and it can take a lot of trial and effort to get things right. The process can be very hard on patients who don't respond well or have unpleasant side effects as you did.

I was one of the lucky ones; antidepressants have always worked well enough to make a meaningful difference for me. The first one I was on had undesirable side effects, but I've been on a regimen for the last 20 years that is still working well for me (fingers crossed).

It sounds like you have a pretty good handle on what works for you at this point - glad to hear it!

I actually tried reducing the dosage once with disastrous effects. That should tell me something.

A psychologist friend has recommended ACT and DBT and I will be trying them. She pointed out that the thrust of Buddhism is to be mindful of pain and discomfort so that they do not affect you, causing "suffering". But pain and discomfort are part of the human existence in one form or another. For me, it is mental health. For others, there may be some other pain that is a challenge.

I am hoping that ACT and DBT help me "handle" my disorder if/when it strikes in the middle of the day while I'm working and meeting with customers.

Wishing you luck in managing your PTSD.

I’ve had lots of experiences and successes working on my depression, but the PTSD was always hiding in the back. Time to address it now.

Typical dose is 900-1800 mg per day. Some say 1200-1800mg.
So you are on the low end of the dosing regimen.

The 450 mg was obviously not enough, you were having break through depression.
It would not be considered a therapeutic dose, so your doctor is doing the right thing.
He is trying to get you up to therapeutic level where your mood is stable with no
symptoms.

If you have side effects let him know and he can decid what he wants to do.

Psychotropic medication dosing takes some time to achieve the desired results,
it takes time to get the patient on the correct dose.

The fact you are having only short daily episodes of depression is a sign you are
going in the right direction. So hang in there. You are doing great.

Your words are very comforting.

my bulimia might be a problem with it.

I hope the new dosage helps you. I've found that head meds are a dimmer switch, not an off one. My symptoms are milder, but I still cycle.

I agree about "dimmer switch". I was used to having suicidal episodes about 2-4 times a year. Once a week is just unacceptable given the meds I'm already on.

I hope your cycles are manageable. My suicidal episodes generally last 6-8 hours. I HATE them. I have this mantra that I run through my head when I have episodes: "This won't last - it's just chemical." I push it through the feelings like a string of sanity holding me to the real world.

I hope you have developed strategies to get you through.