The parable of the sower in Mark 4 (and most of the material that follows) is also copied in the gospels of Matthew and Luke. With a rare bit of transparency, the meaning of the parable is even explained so as to leave little room for interpretation. If people didn't invent so many different versions of the "word" that was to be sown, this story would leave little to the imagination. Even with a lack of agreement about what exactly the "word" is, this can be an encouraging passage for anyone.
The message of the story is really this: You will not be understood and accepted by everyone. You must live by your own guiding principles, and it's a good idea to freely share what motivates you. Some people will be hostile to you if you aren't doing what they think you should do in your life, but you cannot live a fully authentic and satisfying life just doing what other people think you should do. Some people will think your guiding principles are fantastic; they may even be inspired by how you live your life. When push comes to shove, though, they will keep living by their old habits. Some people will be inspired by your guiding principles, and they will strive to live more authentically by their own ideals. Their own irrational fears and beliefs (about themselves, other people, and life in general) will win out, though, at least in the short term. Some people who bear witness to your authentic life based on deep, meaningful guiding principles will be so inspired that they start doing things differently in their own lives. They will dismantle their irrational fears and deepen their own guiding principles, and they will live in such a way that inspires other people -- people you don't even know and may never meet.
You don't have any control over what other people do. You only control your own actions and beliefs. If you are living authentically by a meaningful set of inspiring guiding principles, your life will be like a lamp on a stand -- people won't be able to miss it. You will not see the full impact of your life and actions -- the ripples will spread out further than you can perceive. You don't get to control how other people grow as a result of your own life and actions, but even the smallest moment of authentic action can have tremendous impact on someone else's life. What we do matters, even though we cannot control what other people do with our example.
Uncharacteristically short for me, I know, but this is the core of this parable: Live big. Live authentically. Live fearlessly. Know what your guiding principles are and let them inform what you do. Continually weed out irrational fear and detrimental beliefs about yourself, other people, and life. In other words, be personally responsible in your life, and learn to accept that other people will do whatever they do. Your responsibility is just to live like it matters.