What is "it" that you are not getting?
+1
Teachers come in many forms and teach from many formats.
I can tell you my adult students learn, relatively quickly, can play complete songs, and are having fun with their instrument in about the same amount of time you have invested.
Does your teacher only pound reading standard notation, theory, and scales down your throat? Is it mainly academic? Does he teach you any intuitive skills, how to use your ear, universal patterns that most bass players fall back on, and (gasp!) TAB to learn any of your favorite songs?
I use a combination of all the above. And I let my adult students help me decide what is important and where to put emphasis so they get the most satisfaction from their instrument. (I retain students close to three years on average using diversity and variation in my teaching approach).
I have a couple of adult students, one in his 50s, who never played an instrument before, and they are good enough to gig out.
Do tell us, what is "it" that your teacher is trying to impart to you that you aren't getting?