Positive thinking is powerful when you have alignment or internal congruence - when the thoughts you are thinking are supported internally by corresponding feeling that agrees with the thought. For example, I have experienced myself as a capable landlord. I believe I am good with finding and working with renters. So when I have a thought that I want to find a new long-term renter for one of my units, the thought is supported by an internal congruence and thus my thought that I will find alignment with my internal belief system and the likelihood of finding a quality renter is increased.
On the other hand, if I have had bad renter experiences - I might "think positively" that I will find a quality renter - but deep down I worry I will not because of past troubles. This is where positive thinking will not be as effective as you wish. And it is here you can find ways to move yourself into internal thought-feeling alignment with thoughts that get you more internal congruence...like - well I have learned a lot from my last rental experience and now I know I want in a renter - I will find someone that meets all my new criteria...that is the kind of positive thought that creates internal congruence to get you where you want to go.
Live long and know you are always prospering!