Pessimistic vs Realistic

I was just thinking today...What is the difference between being pessimistic and being realistic.Very often, people say, "This guy is very pessimistic". But, I believe, many a times they are wrong. Let me explain a major difference between these two terms.
Being pessimistic means not knowing oneself.By this, I mean not believing in ones' own capabilities and hence trying to shy away from ones' duties.
Now, being realistic is exactly opposite to this. Being realistic means, according to dictionary, is being practical. That means knowing ones' own strengths and being well aware of his limitations.
Though sometimes a realistic person may sound pessimistic, remember, he is just being practical!!!