Analyzing Scope Creep

Scope creep... as an Instructional Designer you have experienced this before I guarantee it. Why does it happen so frequently? Does it happen with each project? Well, in my experience as a Instructional Designer of 4 months it happens... on almost every project. So, its important to determine how to stop it when it occurs or if it needs to be included, have a process to get it approved.
I am currently working with a Supplier Operations team at a large eCommerce website. I've been asked to reformat and redesign their current training's to make them better overall. Well, I've been working on this project for a bout three months or so and have already experienced a ton of scope creep. The struggle that I have is the project manager I am working with just added a a whole bunch more work but with the same deadline. As a young ID i wasn't sure what to do. So I've said yes. The stress I have now is much greater and I have to do more with less time.
I think moving forward I am going to work with management to have a document that needs to be filled out to get this approved. In this project I am sure I can get it done on time. However, in future projects, if I can't learn to say no I can really get in big trouble. So for me, I have to learn how to deal with scope creep and quickly.