It sounds like the appointment did not go well at all. Shortchanging you for time during your session was not the best way to make you feel you are important. Though sometimes it is hard and things happen, it would be good if your therapist tried to make it up to you by scheduling extra time with you for your next appointment. Maybe you could suggest that to her if this occurs again.
It is good that you recognized the trigger this time when you had your flashback. Each time you can recognize what sets your flashbacks off, you can try to stop them from happening by grounding yourself. With more practice, you should be able to gain some control over them.
It is tempting to want relief from how you feel. Many people feel the way you do when they struggle with depression and feelings of hopelessness. You mentioned that you are aware that there are others who have worse problems than you do. It may be good to take that thought further and considered volunteering to help those who are struggling. Helping others can lift your depression and make you feel you are contributing. It can also provide you with a reason to get up each day. People often report that when they volunteer, they feel they get more out of it than the person they are helping.