Is it selfish to not let others copy off my work that I worked so hard on?

You need to learn to work in a team. Going off on your own won’t help you in the working world. I think the doormat comment above is accurate. Rather than retreating, you need to learn to organize a team, distribute work, hold people accountable, prepare for presentations so everyone knows their role ahead of time, and speak up to the prof if your group members are not pulling their weight.