I took 300 mg of lithium added to my antidepressant about 3-4 years ago. It was added as an "adjunct" or "booster" for my antidepressant. At that time, I was having thoughts of suicide everyday. They weren't at the level of "making a plan" or "gathering tools", so they were more alarming and wearying than terrifying.

But while I was taking the lithium, the suicidal thoughts went away. I stayed on 300 mg for 3-4 months, then my doctor increased the dose to 600 mg and it started causing stomach problems (I think because the antidepressant I take already keeps my stomach always on the edge of having problems.)

I stopped taking the lithium, and the suicidal thoughts came back.

So...I hope that the lithium will do the trick for you. Chronic thoughts of suicide are emotionally exhausting, aside from being potentially dangerous, depending on their severity.