You "cannot do it" might be also a warning to actually not try to fly from a bridge. As such it simply might state the obvious that people do not see due to being very optimistic. I know everybody will tell that determination is king and that you can do everything you want as long as you firmly believe and steadfastly act accordingly. But it isn't true. We humans have limits. Mental and physical limits. And it isn't wise to try to break them. But it might be necessary to explore those limits in your situation. But not at any cost. "You can't do it" is also a personal estimate of what you appear to be capable of. It must not be correct. Also such a comment nourishes your doubt of being able to do it. There is this field where the personal effort and energy needed exceeds what you could apply or have readily available. You probably can apply it but you might experience the cost to be a tough burden. In such a case I try to find an acceptable balance between burden and reward. When thinking about it I sometimes find the warnings to be wrong since nobody knows me that well, or the warnings to be justified since I was overly enthusiastic. Best is to cool down and look at the situation a little later with a bit of a more sober view.