Regardless of whether the investment goes up or down, harboring a grudge / hurt feelings and implementing a multi-decade passive aggressive "I told you so" plan can hurt you psychologically.
You need to learn to let things go.
Yes, your cousin was very hypocritical for bitching about you drinking a beer when he drinks multiple cans of Cocacola every day.
Surely this isn't your first encounter with a hypocrite?
I'm only 34, but one thing I've already learned at this relatively early point in my life is that you've gotta have different standards for yourself than you do other people. You need higher standards for yourself than you do others, or else you will let yourself fall into the traps everyone else falls into. But you need lower standards for them than you hold for yourself, or else you will be constantly frustrated by their poor decisions.
If you're a person that's going to make something out of yourself, and have a good life, you have to accept that the majority of the people you know aren't going to do that, and probably are going to find various ways to delude themselves along the way so that THEY don't feel too bad about their own lives.
Being vindictive will, in the general case, only bring you down by maintaining a source of negativity in your life/thoughts