Well, let's see what Cardinal Newman had to say:
Gloom is no Christian temper; that repentance is not real, which has not love in it; that self-chastisement is not acceptable which is not sweetened by faith and cheerfulness. We must live in sunshine, even when in sorrow; we must live in God's presence, we must not shut ourselves up in our own hearts, even when we are reckoning up our past sins.
Blessed ol' John XXIII:
Consult not your fears, but your hopes and your dreams. Think not about your frustrations, but about your unfulfilled potential. Concern yourself not about what you tried and failed in, but with what it is still possible for you to do.
Our lives are decidedly positive, quite simply because of the nature of the supreme virtues which we are given and called to imitate above all else, those most beautiful gifts of God of faith, hope and love. It was not a 'no' or an indifference that sparked the redemption of humanity, but it was a simple 'yes' by a little peasant girl that granted her not only the title of Mother of God, but granted us the most intimate union with God: Jesus Christ.
Our lives are built on the positive affirmation of 'yes'. There are other entities in life that echo this natural positive; growth being the natural sign of life, for example. We have solid foundations, and from there we can only soar to the heights of heavens. In order to progress in our spiritual lives, I am convinced we need to have a positive,
progressive outlook on the world. How can we ascend if we only look down?
Even such trifling things such as temptation can be turned into a sign of great hope when we realise that we are called more to do good than we are to evil, as Fulton Sheen has said. Temptation then is always an
opportunity to holiness. Love is positive, truth is positive, life is positive. God is positive.
Never lose happiness; never lose your peace. Where your happiness lies, and how we can live in hope and 'being positive' is part of our education in the faith. It is part of our pilgrimage in Christ that we learn that not only does happiness, hope, joy and a positive disposition is of the most natural health to the soul, but that they lie solely in God alone.
Hope that helps.
Edit: When it comes to the subconscious and building up self-esteem or whatever, I personally don't think it's necessary. As has been said, talk to a priest on such issues though I think that being positive is as much a moral issue as it is a psychological one; it is down to one's perspective of both the world around them and of themselves - it is down to your conscience. Pray on it, talk to a priest, and above all: LIVE THE GOSPEL - the GOOD news