Take notes, write down what you have worked on. Companies, technologies, practices. Anything. Doesn't matter.
I've been at this for 9 years, and I've forgotten about half of it, and it's a shame really. That is as all useful information to have on your resume. Now, I think you should have two resumes. One should be quite long, with anything and everything. And you should tailor your resume to each job you apply for.
Not everyone needs to know EVERYTHING you've done. Sometimes a bit of trimming can be a good thing.
That's all, Happy Wednesday.