Surely you've encountered seemingly insurmountable situations in the past. How have you succeeded in the most unpleasant scenarios? Clearly, your ability to overcome these situations speaks loudly for your strength and abilities.
In some situations, you may be able to emulate your past solutions by adapting them to fit your current situation. Other times, you'll be able to learn what not to do from past trials and tribulations.
The past needn't dictate your future. It's possible to reflect without working up old, unresolved feelings. This is a positive exercise. If you begin to feel negative emotions, move onto another technique, which can help you increase self-confidence in your abilities.