I am 16 years old and have been playing piano since I was very little. I have had teachers on and off, but to some non-pianists, my abilities at the piano are amazing. For reference, I can play all three movements of Beethoven’s Moonlight Sonata, parts of La Campanella, and other difficult pieces. Despite others telling me I am a great pianist, I lack technical skills and am not confident in my pianistic abilities. I have an overactive pinky and struggle to play basic Bach Preludes and Fugues. I worry that due to my lack of proper training I cannot truly fix my technique. I aspire to be a composer; I want to be technically capable of playing what I write and writing what I play.
Yes, technical foundations can be fixed even after reaching an otherwise advanced stage. It requires working with a teacher who can help identify problems and guide one toward solutions.
My feeling and experience is that the best approach is to "start over" and have the teacher treat you as though you have no experience at all, playing exercises or pieces they would give to absolute beginners. However, you and they should also require that you play them at the highest musical level you're capable of; meaning, you must treat even the simplest exercise as though it was a masterwork, attending to not just notes and rhythm and technique, but phrasing, articulation, dynamics ... which is to say, interpretation.
Also, consider browsing through similar questions posted on this site. A catalog can be found in Am I too old? Is it too late? Starting, Resuming, or Mastering an instrument