“at the last minute”
At the last minute means immediately before the scheduled time. It is common in discussions of cancellations made just before an appointment.
A2-52 / CEFR A2 / EPISODE 52
Asking about a last-minute cancellation
SceneAt a café. Tina's friend Sarah has canceled at the last minute again.

WORDS & CONTEXT
At the last minute means immediately before the scheduled time. It is common in discussions of cancellations made just before an appointment.
In a row means one after another, consecutively. Here it means that Sarah has canceled their last three plans in a row. It does not mean every plan this month was canceled, and it fits with Tina having watched a movie with Sarah in A2-48.
Cancel on someone means to cancel an arrangement with them. On me adds a sense of inconvenience or disappointment to the affected person. Tina is upset that Sarah has canceled again.
Personal stuff means private matters, and going on means happening. The phrase vaguely refers to personal difficulties when the details are unknown or the speaker does not want to disclose them.
'Cause is an informal shortening of because. Like gonna and gimme, it is used in casual conversation; standard writing uses because. Here it introduces needing time alone as a possible reason for canceling.
Need space means to need time alone or emotional room away from others. Space refers to personal breathing room, rather than outer space. Give someone space means to leave them undisturbed.
One too many times means so many times that it is past the limit. The image is "one more time than can be forgiven": not just many times, but enough to reach the end of one's patience. Because Frank acknowledges how hard it is, Tina puts her limit into words too.
"A message, Tina. Not a book." This short comeback agrees that she should share her feelings but asks her to keep it to a readable length. It is a light tease at Tina, who tends to write long messages (she wrote four pages in A2-18). It does not mean she should keep her feelings to herself.