The phrase "modesty forbids/prevents me from doing something" often implies reluctance or humility when opting out of a situation for personal reasons. Other synonyms to express this idea include "reserve," "bashfulness," "shyness" or "reticence." Each of these terms suggests a hesitancy towards something and/or a lack of forwardness that steals your confidence when speaking or acting. You may also use the phrases "I'm uncomfortable doing that," or "I'd rather not," to convey the same message without using a specific term. Ultimately, it's important to feel comfortable and confident in your decisions, and to know that expressing your reservations is a valid choice in any situation.