It is a mistake to think that by fearing or being unhappy you can progress. Fear is always a feeling to be rejected, because what you fear is just the thing that is likely to come to you: fear attracts the object of fear. Unhappiness weakens the strength and lays one more open to the causes of unhappiness.
One can be quiet, happy, cheerful without being all that in a light or shallow way—and the happiness need not bring any vital reaction. All that you need to do is to be observant and vigilant,—watchful so that you may not give assent to wrong movements or the return of the old feelings, darkness, confusion etc. Not fear, but vigilance. If you remain vigilant, then with the increase of the Force upholding you, a power of self-control will come, a power to see and reject the wrong turn or the wrong reaction when it comes. Fear and unhappiness will not give you that. It is only by this vigilance accompanied by an opening to the supporting and guiding Force that it will come. What you describe as a capacity to choose the right and the feeling of strength or power that can stop the wrong movement and take the right one as soon as it recognises them, is just this control and vigilance. It is by this control and vigilance supported by the Force that you can prevent the love and devotion too from being mixed with or replaced by selfish desires and impurities. The more you open, the more this power will increase in you.
Certainly, if talking about old things or excessive speaking brings the wrong reactions at night, it is better to abstain from such talk or speech. Here also control is the thing you have to develop.
Ref : Letters on Yoga Vol. IV