Imagine a situation when you spoke to someone with the best of intentions but it turned into a misunderstanding. Now you can justify yourself thinking that your intentions were good and conclude that the misunderstanding was due to some problem with your friend. But it may have been that you had not taken into consideration some sensitivity on your friend's part which you knew earlier. When this comes into your mind now as you think about the incident, you can still avoid a sense of guilt by thinking that it was your friend's fault and that he shouldn't be so sensitive. What has happened here is that in effect you have chosen to stay where you are in spiritual development and make no progress.
Justifying ourselves and passing the blame to other people is one of the first results of the Fall (Ge.3:12,13). When we justify ourselves we may not be even telling ourselves lies because we may be 'truthful' depending on certain particular facts we look at. But the problem is that there may be other facts which we are conveniently forgetting or putting aside from our consideration.
The reason why this happens may be that we are very insecure about God's love towards us. Even though we know in theory that God's love comes to us from His unmerited favour towards us, in practice we may be still thinking that whenever we fall or make a mistake His displeasure will kick in! Another reason may be that our self-image is so poor that we don't want to let it get further dented.
The joy of the Lord is our strength (Ne.8:10). When we are feeling joyful, this is fine. But when the circumstances of life are making us feel all confused or lost, we would need to draw our strength from Him by consciously going to Him, recognising His love and acceptance, and allowing our joy to be restored. When He is for us, why do we worry about those who are against us (Ro.8:31)? Why should we worry about what might come to us in future when we have One who holds the future in His hands (Ro.8:28)?
When we draw our strength from the Lord, we can afford to look at our failures and shortcomings without losing courage. We know we are far from being perfect and that God is at work in us to make us more and more like His Son (Ro.8:29). The way this transformation takes place is when we acknowledge our lacks honestly, go to Him and draw strength from Him to do things better next time.
Sometimes, when we are sensitive to the voice of the Holy Spirit, we may be able to know that something we are thinking of doing is wrong and avoid doing it. But more often what happens is that after we do wrong we come to know of it. It is only the next time we can avoid that mistake. This means that we should be able to look back and acknowledge that what we did was not perfect, recognise the mistake, learn from it and do better the next time. To be able to do this, we should accept as a rule the possibility that we could be wrong any time and be willing to take feedback.