When creating material you can give yourself permission to use your own work, but you might not have permission to submit it to an academic institution. Plagiarism isn't just about copying the work of another person.
Plagiarism is claiming to have done work when you actually didn't do it. If you take your old recycled work and represent it as new work, then that's plagiarizing yourself.
It seems a bit unfair because you actually did the work in the past and now you're using it. Just remember, when submitting new work, teachers and professors expect a whole new creation from scratch.
Many corporations reuse their own work when submitting business models or proposals. However, the business industry and the academic industry have different rules.
Here are some ways to avoid self-plagiarism:
Citefast.com is an online citation creator that will save hours when citing material.
Easybib.com is another online citation creator that will save hours when citing material.