The most successful people in the world mentally rehearse what they are about to do - sink a putt, act on stage, talk to venture capitalists - before they do it. Envisioning a successful future is indeed very important to achieving it. Before you went to Asia, you probably mentally imaged it as enjoyable. To say that positive visualization makes you weaker is therefore nonsense.
However, visualization is just the first step. Visualization alone will not help you succeed in life, nor will it prevent someone you love from leaving you for one reason or another related to them not you (your single mother).
Your friend visualized being a mother continually and became one; that was in her control. I never really visualized having kids, so I never really ended up with kids. Thats how it works. That and lining up your unconsciousness to work on the problem while your conscious mind is off doing other things.
Having said that, I'm not going to visualize making thousands of dollars on this post because although it is a little more nuanced than your post, factors other than relevance and truth determine the Steem world. Because I fail to visualize; the Steem will fail to follow :)