Pick something and make that thing until you do it really well. Do it until you are sick of it.
Pillows work, so do pajama pants, tee shirts if you want to work on stretchy material sewing. Maybe even stuffed animals, felties are great for being able to focus on the technique without worrying about the fabrics all that much. Heck Christmas ornaments would be great hand sewing practice, and you can give away the best ones and wow your friends & family.
When I was about 13 and given free reign on my moms Singer (allowed to sew without adult supervision), I made about two dozen of the same stupid blouse, in different colors. Then I made bloomers, hey it was the eighties- they were popular at the time. Then I got on a circle skirt kick and made a bunch of those. Then I got really ambitious and made a Renn faire costume, chemise, petticoat, overdress & cloak. It took me months but turned out pretty good.
Then I signed up for sewing class in school. My teacher wasn't about to let me get an easy A, she offered me a dozen patterns that would actually be challenging, I chose the bikini. It turned out great, but I'm still not a fan of stretchy sewing.