There are three possible saves that ofp can restore to. One is the position when you last exited the mission alive, that file is called 'continue.fps' another is the one Save that you get in the mission (that file is called save.fps) and the other is the last time the savegame command is used (this file is called autosave.fps). When the mission restarts is always looks for these files in a specific order, which from memory is 1. Continue; 2. Save and 3. Autosave. It always loads the first one it finds.
So to get to the autosave position you need to resume the mission and it will put you to either the Continue or the Save position (which ever it finds first), then press the escape key and select Retry and that will take you to the last autosave