As others have said, there are ways around it. You may not have had a formal trainer but by being in the boxing gym and training with other fighters, light sparring, hard sparring, working on certain drills to further sharpen instincts, being watched and advised by some trainers along the way on garnering knowledge that's effective for the ring etc. You could have a lot of knowledge but you may not know how to breathe properly when you're throwing and landing punches, you may not realize that your ring positioning is completely off so the shots you are thinking of throwing will be completely thwarted by your opponents better angle who can either land the better shots or force you further out of position and take advantage. You may keep your hand low after the jab despite knowing in principle not to.
The point being that it's incredibly hard to do, we don't see many examples at all in history of this. Besides the technical aspects, trainers will know your boundaries, oversee and push it accordingly. They also help you live accordingly to a particular philosophy and makes sure that the philosophy works for you, so you train with purpose. They'll be the third party observer that is committed to looking at your blindspots etc.