Power struggles, increased pressure and ticking clocks, moral dilemmas and temptation, loss of control, ego-related conflict

"Ultimatum" is a Latin word meaning "last one." It's a final demand that, if not met, will result in serious consequences for the character.

In romantic relationships, an ultimatum is often an order for the character to give up something (a job, dream, hobby, or another person) or risk losing the person making the demand. Sometimes, instead of asking the character to give something up, the other person demands that the character do something they don't want to do. At work, this might involve the character having to violate a moral code, break a promise, or marginalize someone else in order to keep their job.

While ultimatums have a largely negative connotation, keep in mind that they're not always bad or unreasonable. Demands to stop abusing drugs, be an involved parent, get to work on time, or stick to the rules of one's parole are legitimate ones meant to establish healthy boundaries or help the character make better choices. But that doesn't negate the stress and conflict that result when even a well-meaning ultimatum is given.

The situation keeping the character up at night
Trouble focusing at school or work
Other relationships suffering (because the character is keeping secrets, they're taking out the stress on their kids, etc.)
Minor health issues, such as weight loss, stomach upset, headaches or fatigue
Dragging things out and prolonging the agony (through stalling, avoidance, denial, etc.)