Definitely an issue with the oil or more serious gear problems. The fact that forcing it causes the engine to stall means the transmission alone is bound up enough to not spin. I would fear the transmission is toast. Like others say I would second the notion to first check the current
oil level, if you haven't, and then change the fluid. If the current level is fine and the fluid is not totally black, it is probably not the fluid and is something more seriously wrong in the gearbox
. If you are lucky, you are momentarily low on fluid and adding the proper amount fixes it. I would not be too hopeful about that.
As a diagnosis, what is it like to shift with the engine off? It should be smooth. Good luck.