It is what the voice of conscience says that is crucial. The optimistic voice of conscience tells us the best while the pessimistic voice points out the worst. We all have such an inner voice, and it is more or less optimistic. If this voice is already positive, we can strengthen it. If the voice is negative, we can change it.
Hard work is required, however.
Being negative is easier, but it is not as healthy. The key skill in optimism is talking sense to yourself. Learn to catch the pessimistic voice in action and challenge it. Don't let it drone on and on about how it is all your fault, how you have ruined everything and how nothing will never get better. Start paying attention to what you say to yourself.