1. Forgiveness: I can't forgive this, or him, or her. But God commands forgiveness (Matthew 18:35). So we can, in fact, determine to put aside anger and soften our hearts with the knowledge of the gospel of grace, and act as though the wrong had not happened. When we say we can't, we mean we won't; that we want to hang on to our anger, our bitterness, our "right" to get even, under the excuse of being "unable."
2. Difficult truth-telling: I just can't tell him the truth. It would destroy him/me. God tells us to "speak the truth in love" (Ephesians 4:15,25). Often, we are excusing cowardice or pride under "can't." What we really mean is: If I tell him that, he may not like me anymore. I would be humiliated. He would be upset. I won't risk that cost - I would rather disobey.
3. Temptation: I can't resist doing this, though I know it is wrong. We must be careful here, because sin has addictive power - it is true that we may not be able, through sheer willpower, to stop doing something by ourselves. But we can get help, admit our problem, humble ourselves, cry out to God for mercy and transformation, become accountable. God always gives us a way out (1 Corinthians 10:13) - no sinful thought or action is inevitable and irresistible. If we don't, it's likely that we would simply rather keep sinning in that way, excusing it with our "inability" to do anything else.
