I am stuck on a mission that I need to complete for the payout or go broke. I have tried to start this mission for 4 days. I enter the mission loading screen but it continues to either not load or crashes....
HELP

I've heard of that happening with corrupted saves. I don't suppose you have another save you can load from a little farther back, do you? If you aren't playing Ironman, then you should have a whole lot of autosaves to choose from even if you've never manually saved. If you are playing Ironman, then you should go to the Paradox forums and report the bug, you may get better help there: https://forum.paradoxplaza.com/forum/index.php?forums/battletech.994/

Not playing Ironman, but, the last mission was extremely difficult and I really don't want to play a mission again. It was rated as a 2 skull but played more like a 4 or 5.

Well, going back worked AND it was much easier this time. Less injuries and repairs.