I started the last mission to invade the Vatican but without enough medicine and throw knives. So now I'm stuck cause I'll die if I proceed yet I can't return to previous mission/checkpoint.

How?

I had 3. Managed to do it anyway by using the pistol and throwing knives from afar so less guards will swarm you before you open the gate to where the monks are. Also pickpocket the dead guards and there was medicine as well.