You've basically answered your own question:
> My guitar is fine
> I've ... had a lot of lessons.... So I feel better about self-studying guitar
> I've also been singing ... without any actual instruction.
> Help!
So what's the difference between the thing you feel better about and the thing you feel worse about? What did you learn from your guitar teacher that you wouldn't have learned alone? It won't be the same for voice, but it'll be analagous.
More specifically, for learning, feedback is best when given immediately. A voice coach would be able to give immediate feedback, rather than you (potentially) discovering it later on on your own.
Also, an experienced coach will be able to match your weaknesses with exercises specifically chosen to address the challenges you are facing. Exercises appropriate for your current level and practicing the skills you need.
Finally, there is a lot of subtlety in vocal technique. There are tons of things that most people do not notice in self-observation or through recordings that a coach can pick up on and help you through.