How do you stop being easily affected by others?

From what you've said, I'm guessing you have low self-esteem, very little self-belief, and many insecurities. This is not something that is easily fixed. I would suggest therapy to try and identify the root of these issues. Often, they originate in childhood from various negative experiences or some kind of trauma. You can also trace the origins yourself by identifying when and how you started feeling the way you do and tracking back the progression of your issues.Once you know where they came from, it's a lot easier to process and understand those past experiences in order to change your perception of yourself and the world around you.

For example, if an uncle told you as a 6 year old that you were dumb, you might have internalized that and believe yourself to be unintelligent. Looking back at that as an adult, you might see that the uncle who told it to you was going through an acrimonious divorce at the time and therefore taking out his own problematic issues on an innocent boy because that made him feel better about himself. That understanding would allow you to perceive the situation differently and change your thinking and behaviors in the future. 

You might find some guidance in "A Warrior's Path" by Robert Trivino, and "Understanding Yourself" by Mark Prophet.

I only hope I can explain this correctly to you.  The human race is about 90% dominated by the side of the brain that stores and keeps knowledge about statistics, sciences, and mathematics.  These are the genius types with great new inventions, and creations, etc.  But the other 5% to 10% of humans are dominated by their emotional side of our minds, by the arts, by psychology, by the mind itself.  I  am like this, and I suspect that you are also.  Poets, great authors, musicians and ppl who work with their hands are also in these groups. Artists and poets, and people who live in a beautiful world that the other 90% of humanity can only wonder at.  Yeah, sometimes I feel left out or omitted, but you know I would never trade my own special VISION of Life for any of the other ones.  Being emotional has its downside, but for a change, just feel and consider yourself blessed indeed, to be among a very special group of people.