Please, please, please add a manual save function

I just played an entire level on legendary difficulty and after leaving the game, was sent back to the beginning. This is not the first time I’ve had problems with saves in this game, and I have to say, it is discouraging to the extent that it makes me not even want to play it anymore. I’m aware of the difference between saving check point and saving progress. The point is, after playing through an entire mission, one obviously will have crossed at least one saving progress point. For some reason that I can only assume is a glitch, this does not happen every time. So, one solution would be to provide a damn manual save function! I know I’m not the first person to suggest this. How hard could it be to add this functionality, or at least do something to address the save points in this game! Otherwise, people are going to quit playing your game because they can’t be bothered to be wasting their time.