Tell me this gets easier. Please. I drank the kool-aid.

I am pre-finishing my current project so when I am done with assembly all I will have left to do for finish work is the exterior. I have made my first dumb mistake. I still believe in the idea of pre-finishing, and I came at with no small amount of fear of screwing up. I planned and planned and planned, and I have screwed up.

I am building a wall hanging cabinet. The inside back wall is painted one color. The inside walls, floor and ceiling are bare wood with oil and wax. The exterior case will be a different color. I got the dovetailed case glued up, and those four pieces oiled and waxed on the interior. I got the bottom piece of the back wall painted and installed - but I forgot to cut a tongue or a groove in it to mate with the rest of the pieces of the back wall.

I can pull this put of the fire by cutting most of the groove I need with my plow plane, but I am going to need assassin level work with my dovetail saw where the plow plane can't reach. I only got assassin level skill with my dovetail saw about three days per month. You see the problem.

What mistakes have been made that I can learn from the experiences of others before I do them again? What mistakes am I doomed to make on my own regardless of your various remonstrations?

What is a good tape to use to cover tenons when prefinishing table aprons? I see that one coming.

How do you keep finish out of the mortises in table legs so you have good area for glue up later? I see that one coming.

What is going to cause me (Navy) to string together profanities in a way God almighty has never heard before, and how do I prevent that?

Thanks. This is a good idea and there was going to be a learning curve, but I am committed to getting this right.