Question

If I'm playing a mission and I quit the game, is there any way of continuing where I left off? Thanks!

As far as I know, No.

So you just have to start again?

Yeah......

Well, you can go back to the specific level of the mission that you were on. But as far as going back into the middle of a level, no you can't. Unless you have some friends who you were playing with and didn't leave the mission. Then, you could join their game again and spawn at the beginning of the level.

yeah that's the only way so far. You should be able to place session markers. :)