Mac running slower and slower despite disk cleanups

I have an old MacBook Pro (13inch, early 2011). As it has slowed down over the years, I have deleted at least 10 big applications to free up space. All my old Adobe products have gone and now I'm on Creative Cloud with just a few essential programs downloaded. I've gotten rid of things like GarangeBand, Final Cut. I've cleared out caches, offloaded many video files to external HDs. And yet my HD always fills right back up and I can often not even open a photo in Photoshop, because the scratch disks are full. My OS is Sierra. Why, despite these repeated cleanups, am I still out of space and the machine so slow? I used to store so much more on this machine?!