There are even books written about and affirmations you could do, to make you positive. This "must" is already decided that you are possibly negative.
Positive thinking is superficial if the underlying mass of negative thinking is covered. Positive thinking will be a resistance to not experience having your negativity.
Positive thinking happens automatically if you're rid of your negativity, such as your hidden negative intentions behind your so called positive comments, evil thoughts, jealousy, pessimistic attitudes and the shame and guilt about this with the fear of being seen with all that shit you are covering.
Because your negativity is still there if you hide it and don't want to see it. And this will always be somewhere in your appearance or erupt when you are on your guard, in a response or getting a disease. The harder you want to be positive and nice, the more likely that this is leading to a crisis. Besides, life does show how you are, look at your life now, how rich and full is your life or what nasty things are coming at you.
The only way to be completely positive is bringing negativity in your awareness and begin to see the connections where the negativity is linked to. At the same time taking responsibility and acceptance for that part of yourself.
If your "dark side" can come to light they can transform themselves.
In order to "must" be positive is also linked to the outside world, because you have to be focussed, to be good, to belong to a group or other person, th need to be appreciated, so you can't use negativity.
As long as you only remains focussed on the world around you, you forget your own inner world.
And that is the point if you want to see your positive and negative sides. When you enter into your inner world, you can see your own entrenched beliefs that affect your life negatively. You discover your inner images wish its childish emotions, formed in your childhood, which influences your life.
When you realize that your inner world is the real world, because that is what you take constantly with you - and that de outside world is a temporary illusion, because everythin you do in the world decays. You take yourself with you forever.
It is better to delve into your own world to know yourself better, your own autonomy and your own power and intuitive knowledge. In fact you have all the capabilities already within you to solve any problem you have. To learn confidence in your own resources. That you can stay in the outside world without being dependent from this world.
You are going to discover the flow of your inner journey, where it impedes and when it unfolds when you experience your spontaneity, creativity, love and happiness. Important is increasing your awareness what takes place internally and learn from what is coming to you as a mirror from the outside world. Meditation regularly in yourself to discover your infinite universe, which you first experience as silence void. In this way you become filled with your own truth, your own reality.