Your favorite artist just released a new album. You are excited for their new material and eagerly buy the new album. When you sit down and listen, you discover that half the songs were from their previous albums. You expected new material, but the artist just renamed and re-released older songs. You are rightfully upset. Your audience, whether that is your professor or your fellow students, has even reason to believe that what they are reading is new and original. Just like when you listen to an artist's new album, you correctly assume that the album contains entirely new material. It is fine to repurpose your previous work. If you choose to do this, but talk to your instructor before you do about what the appropriate amount is.
Self-plagiarism in real life:
Johan Lehrer used the material from his previously published articles and sold them in his book. Lehrer resigned from his position at the The New Yorker because of his self-plagiarism and other ethical violations.
Avoiding plagiarism, self-plagiarism, and other questionable writing practices: A guide to ethical writing