Without a specific chapter reference, this question is a bit difficult. Isabel is frequently depressed, and she has reason to be depressed. The book starts out with her and her sister being freed slaves. Unfortunately, nobody is willing to honor that agreement from her former master, and she is quickly sold back into slavery. That's bad, but it's made worse by the fact that her new owners, the Locktons, are horrible people. Madam Lockton physically and verbally abuses both Isabel and Ruth. Perhaps Isabel wouldn't be diagnosed with clinical depression at this point, but she is for sure not happy.
I think the lowest point in Isabel's life in this book is when she discovers that Madam Lockton has gotten rid of Ruth, and Madam Lockton refuses to tell Isabel any specifics about it. Isabel is so upset that she threatens Madam Lockton, and Madam Lockton has Isabel branded on the face. At this point, Isabel feels that all hope has been lost. She's been branded, she's lost her sister, and she's delirious with fear, pain, and sadness.
A key point that I would like to make about depression is that someone doesn't just "snap out of it." It's not an instantaneous change from depressed to not depressed. The shift is gradual, and it takes time and effort. This is the case for Isabel as well. It begins with her healing from the branding. Curzon enlists Lady Seymour's help for this, and despite being white and rich, she is the complete opposite of Madam Lockton. Lady Seymour treats Isabel with kindness and looks for ways to help Isabel. Lady Seymour even attempts to find out what happened to Ruth. She is a key component to getting Isabel out of her depression.
Another key is Curzon. If I'm honest, Curzon doesn't intentionally do anything to help Isabel's depression. He gets thrown in prison, and his living conditions are atrocious. Isabel isn't willing to let her friend rot in prison, so she makes it a goal to bring him food and try to keep him as healthy as she is able. This helps her depression because it allows her to focus on something and someone other than herself and her present condition.
Third, Isabel begins reading Thomas Paine's Common Sense. Many of the passages give reasons why the colonists are justified to seek independence. The work explains that people with wealth and influence were never meant to rule over those who lack it, and the message gives her hope.
Finally, I think the one thing that really pulls Isabel out of her depression and gives her a reason to keep fighting is the news that Ruth is still alive and still owned by Madam Lockton. Madam Lockton couldn't find a buyer for Ruth, so Madam Lockton moved Ruth to the Locktons' property in Charleston. This news is discovered in chapter 43, and the remaining two chapters are about Isabel's escape, rescue of Curzon, and flight from the city. The book ends by giving readers the impression that Isabel will stop at nothing to get to her sister.