"Do YOU think that selfish people are happier, mentally and psychologically healthier, and more successful than those who ARE NOT?"
I think that people who are honest with themselves and others are happier mentally and yes, psychologically healthier but not necessarily more successful. For example, I always thought that I was too selfish to have children. I liked doing what I wanted to do, when I wanted to do it; spending my money frivolously rather than on items of necessity or managing a budget; even sleeping in until noon if I wanted to. I knew this about myself, but I was also honest to myself which, I believe, has made me a better mother. Granted, I think that anyone with a brain finds moments of self doubt, mental or psychological distress and those that take it to the extreme can find themselves alienating everyone around them, losing their job, etc. but I don't believe that any type of self being such as selfishness or the power of positive thinking can affect anyone's financial success.
I do think that selfless people tend to be mislead and manipulated easier than those that are selfish, thus allowing self absorbed people to claw their way to the top without a care for those they stepped on along the way. In short, I think that it's a matter of how far a person is willing to go in order to get what they want determines their 'success' and this in turn leads to them believing they are happy due to the income they can bring in.