It is not possible to set out to make a positive difference in this world. This is for the simple fact: you have no idea what effect your actions will have.

You have no idea whether your action will result in something that ultimately ends up being positive or negative. You also have no idea what making a positive difference really means. You may think you know, but "a positive difference" is a very relative term.

Plus, you can guarantee yourself being completely wrong by having a strong conviction in your belief that what you are doing is making a positive difference. It is irrational to unwaveringly believe in something that you cannot possibly know the outcome of. Humans are fallable, and any idea or interpretation that they think of has a chance of being completely wrong. You have to accept that fact and thus you cannot have an unwavering conviction that what you are doing will make a possitive difference.

Thus, in summation, it is not possible to set out to make a positive difference in this world.