It was very difficult when I was younger, but can somehow do it now :) But it takes practise 

Sometimes you have to turn down others invitations as everyone has their own life and business. Please Don't think too much all the time, sometimes you should consider things from your own point of view.

Don't think too much, and just turn it down,

It is not that hard to turn down invitation. Because, that need traning to turn down.

Some of people will think directly that will not be manner. They want to share something with you forcedly.

That is the fear factor you dare not turn down. So you should have courage to turn down it without thinking too much.