Not only does this immediately discourage, it also provides a convenient avenue for me to direct anger toward someone else. I have to do X, and can't because you still didn't do Y. But with God, whatsoever things the Lord speaks are possible, even if you never do Y; I must simply be still, and wait upon the Lord.
I can offer no great insight at this time; I can only say that it has already been helpful for me to notice when my thoughts stray toward the "I can't" category. Recognizing those thoughts as coming from Satan is probably half the battle.