I think if you can afford lessons, it would be a great investment. You seem passionate about playing the piano.
It's hard to have a general perspective over your development when you are in the middle of it, I have experienced many musicians can get a little worse period wise when focus ends up on small things instead of the whole picture. But it is important to focus on details to develop your skills and understanding. Perhaps you don't think you focus alot on details, but I'm sure there is some sort of thing holding you back. But have you become a better musician than some time ago? I'm sure you have! But today you see things in your own musicianship that you wish to improve and/or change and the old truth "the more you learn the more you realize how little you know" applies when you feel like you hit a wall. Today you can see faults in yourself that you didn't know existed 1 year, 6 months ago or whenever you felt like it was going good.
To be a musician is a lifelong study, and for some maybe most, to learn how to handle the feeling of not being good enough.
I'm sure, that you will get where you want to go if you keep working.
Also, are you playing something you really want to play? Will it give you a GREAT satisfaction when you are done, will you be proud of it?
If not, go get something to work on that you will enjoy more than anything else. (Within reason ofcourse, wouldn't want you to damage your hands or develop poor technique).
If you get the teacher, ask her/him for repertoir suggestions, write a list of things you would want to play, and don't feel down if s/he tells you it has to wait, it's only a matter of determination and time.