The Projects section in the sidebar shows both Projects and Portfolios and you can’t make it only show Portfolios. You can choose one of three sorts:
If you only want to see Portfolios, I actually recommend you collapse Projects and/or drag it to the bottom of the sidebar out of the way.
Go to Portfolios and create each Portfolio you want, adding the desired projects into each with the Add work button in the Portfolio, then “star” the portfolio:
Then the portfolio will appear in the sidebar in the Starred section; you can drag and drop its contents to reorder.
You may also want to vote for this request (purple vote button at the top of that thread):
Thanks,
Larry