If you catch wind of others belittling you or your goals, decisions or perspectives, then keep in mind that they are not you, and are not called and created to do what you're called and created to do. These are probably the same kind of people who complain about more than they praise, and who still need to figure out what it is they were made to do - don't let their lack of pursuit slow yours down.
People who are never short of negative opinions about everyone and everything have forgotten what its like to go after anything that warrants any opinions at all.
Don't let the ones doing nothing prevent you from doing something.
There will also always be those who can't seem to see the good in anyone. When someone 'finds fault' about other people or things without any justification other than their own insecurity and fear (or envy), and you're caught in the conversation, it can be difficult to chime in or cut them off and suggest another perspective. Then again, you don't need to. You can't really help someone develop a positive perspective when they are happily staying unhappy. People stay happily unhappy because its easier, safer and less intimidating to keep doing what they've always done, even if they're desperate for change. A byproduct of that kind of unhappiness is an unyielding distaste for anyone brave enough to pursue happiness.
Its one thing to have different opinions, especially when they are thoughtfully discussed and articulated, but if someone persists to fill up your time together with nothing but pessimism or negativity, then it might be time to think about cutting off your relationship, or at least cutting back the time you spend on that person. If you run in the same circles as someone who has spoken unjustly about you behind your back, be the bigger person and treat them with the love and respect they should have been treating you with. And, if the person who speaks negatively of you isn't even in your life at all, then let it go and don't bother defending yourself - confrontation would be a waste of the time that should be spent continuing to move forward in all that you were made to be. Now, go be it!