Warren Buffett, famous American businessman once said “The difference between successful people and really successful people is that really successful people say no to almost everything.”
If you have problems with assertiveness and saying "no" then this book is a must-read.
You'd be surprised how much time you "waste" by casually hanging around with friends every week. Much better to focus on the important things you need to do and have some more "focused" friend-time, e.g. by throwing a party once a week. You'll see that you will a) get your stuff done and b) have even more intense experience with your friends.
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