@edgarblythe,
You have every right to leave the Democratic party... and to leave both parties. It means that you are giving up any political power, your vote no longer means anything and you have no ability to influence your government. But that is your right.
What is silly about this thread is your apparent believe that there are enough people who agree with you to form a third party.
The problem with ideological purity is that it is very exclusive. Someone might agree with you about GMOs but disagree with you about abortion, or guns, or the use of military force.
Coalition building means compromise. This is why a party built on ideological purity never succeed.
You are making the decision to withdraw from real-life politics (fantasy utopian politics doesn't count). That is your decision.
But many people choose to remain engaged. They promote their issues, work to elect candidates with a real chance of winning, raise money, and work to change hearts and minds.
Bernie is an great example of this. I have great respect for him.