I was not paying attention and ended up loading out with the wrong equipment. Now the game will not let me return to briefing and start over. Have I screwed the pooch and am stuck with wrong equip?

Some times this games causes you to CUSS!

How about going in: OPTIONS/SAVEGAMES/MISSIONXX*_L.DSL

*(XX)Generally a number between 01 & 10

Blame yourself, not the game.

load the first save for they mission it will let you start the mission over and get new things