I second what Wibble stated! A mentor that is skilled and can communicate what you are doing correctly and doing wrong is paramount. Also, have someone video you while you are practicing your skills. This is important so YOU can visualize what mistakes need to be corrected. You don't want to reinforce bad habits.It's the mentor that matters. You need a skilled technical diver to mentor you. It could be called a course, but it's more one-to-one mentoring for a couple of sessions.
You don't need a certificate, the skills speak far louder.
Cracking the core skills, buoyancy, trim and finning does take time to master. Having a decent mentor will really help; literally showing you what "good" looks like. You then need to go off and practice which may take some time (e.g. back finning!)
You can sometimes find GUE or Tech instructors that will work by the hour to help with skills.
Keep at it you are on the right path!