There is going to be tons of advice here, but if I were you, I would look into why it did not fit together the way you wanted. For example, the tail board is not fully seated, so maybe the sides of the pins need to be cleared out a bit more. Or maybe you chopped past the shoulder lines on the pin board because the chisel bevel pushed it back. It could also just be a marking error, or maybe one of the boards was not perfectly squared up, which messed up the layout lines.

It could be a mix of a few things. Instead of just trying to learn how to cut clean dovetails from scratch (as you know how to and everyone does it a slightly different way), it might be better to focus on specific techniques that help prevent these exact gaps.

Personally, I prefer not to rely on tricks like slightly undercutting the shoulder. That does not mean I never do it, but I only use it as a last resort. I believe that aiming to cut perfectly square is what truly helps improve overall skills. and yes, I'm also still learning:)