I recently bought a P365X and, though I haven't gotten used to the trigger as of yet, I like the gun. The biggest problem has been the difficulty in loading the 12 round magazines. Even with a loading device the mags are next to impossible to load. I've been loading various mags in 9mm and .45 for many years and these are by far the most difficult. Is there a trick to it, do they 'break in' or is there a special loading device to make it easier. I have a Browing HP, a Sig P239, a 9mm Shield M2, Hellcat Pro 9mm, Ruger Max9 and several .45's and none are even close to the difficulty of loading these mags

I have the same gun and the same issue with loading the mags. I shattered my right wrist years ago and lack strength in my hand. I used the Magula to load them at first. After being fully loaded and cycled they loosen up a bit to where you don't need the device.

I did read where there is some possible binding with the follower that wears in and reduces the force to load. Mine broke in fine over time.

IME, the last two rounds in any of the 365 magazines, (365, 365X/XL, 365 Macro) have all been incredibly tough to load right out of the box.
However, they have all loosened up over time and use.

Get a maglula loader and place the mag base on a hard surface. They are pretty easy to load using that technique

I use this  and it works good.

This it really does work well.  As mentioned all P365 mags the last rounds are difficult to load but get better the more you use them.  If you look where the last round is in the window you can see how much compression you are getting on that spring.

I use a similar loader that came with my Hellcat Pro but it is still very difficult to load.

I broke down and ordered a Maglula.

Best 30 or so bucks you ever spent especially if you shoot a lot of pistol.

I broke down and bought the MagLula and yes it it amazing for loading mags.

Next question is where can I buy extra mags for the P365X.

I have gotten most of my mags from GunMagWarehouse been the best price I could find.  They sell all the variations I have been buying a lot of the 17rd mags lately and got two extra XL/X mags with my new XL and I got two with my X.  Now I only need two standard P365 10 round mags when I get a standard frame and probably a few more 17rd mags.

get them full.

Leave them that way.  Shoot um.  They will free right up.