Whether you think you can't handle being rejected by a love interest, or you're convinced you can't deal with the discomfort associated with making a career change, second guessing your ability to tolerate distress will only hold you back in life. In fact, the more you doubt your mental strength, the more important it is to prove that you're stronger than you know.
Allowing your exaggeratedly negative self-talk to dictate your behavior will prevent you from reaching your greatest potential. Just because you think you're not mentally strong enough to do something, doesn't mean it's true. You're likely able to tolerate much more than you think.