DIfference between Wellspring of Life and Mortis?

Wellspring of Life as seen in CW season 6:
http://starwars.wikia.com/wiki/Wellspring_of_Life

Mortis as seen in CW season 3:
http://starwars.wikia.com/wiki/Mortis

both were crazily powerful planets of the force. Was there really a difference between them or did they represent the same basic thing- gargantuan force nexuses?

Wellspring of Life is the origin of the living Force.

Mortis is the embodiment of the cosmic Force and in a higher realm.

Big difference, tbh.

Yeah, the WoL = living force; Mortis = cosmic force. The former feeds/powers the latter.

Why couldn't it be just God?

The Wellspring of Life is not the origin of the Living Force. It's the foundation of what connects the Living Force to the Cosmic Force.

Those two ideas are not exclusive. It's both.

So does that mean that the Force Priestess is the embodiment of the Living Force ?

They "embodied the connection between the living Force present in lifeforms big and small, and the larger, timeless cosmic Force" (source: SW.com).

The Living Force is the energy produced by living organisms, so the Force Planet is the origin of the Living Planet given it is where the first life (midichlorians) originated.

It's less clear from a canon perspective, but from a legends perspective, the Wellspring of Life would have predated Mortis. The former is where life originated, and where the force would have theoretically been born; the latter is the home world of the celestial beings who had the greatest impact on the force.

And I am to the show like:

You all need Jesus.