Layer masks and gaussian blur

I think you have a problem with layer order. Try blurring the top layer's horizontal edge since it is on top; that's one possible solution. You can do this using the brush tool: draw a line with a large soft brush and paint with black on the mask. Take a look at the @pixxxelschubser first example; that's what I am suggesting.

I am not sure that I am completely understanding what is happing with your project.  Could it be that there is no content in the are you have an issue with.  You can test this by Ctrl clicking the layer's Eye icon to load it as a selection

And then Ctrl click the layer mask.

Or you can Shift click the Mask to turn it off and reveal the full content of the layer.  Note the red X on the mask that tells us it has been disabled.

You can unlink the mask by click the chain icon between the mask and thumbnail which lets you move either the mask or layer independently.

One problem with layer masks is that we can't make them into Smart Objects (SOs).  SOs let you edit their contents non-destructively.  If you apply Gaussian Blur to an SO layer and don't like the effect, you can double click the SO thumbnail or filter (which appears under the layer) and change it.  You can do this as many times as you need.

What we can do non-destructively to layer masks, is adjust their properties, as below.

If you keeping everything as clean edged rectangles, then the Frame Tool is excellent, and too often overlooked I suspect.  However, you don't need to keep the different elements clean.  They can overlap each other, and even stray outside the montage if you give the document a white border.

This was a brief for Wellington University's Multicultural department.  The images came from years of covering my local Multicultural Festival.  The downside of this it's a lot of work, and it's easy to get confused, but it's a lot of fun and good experience.

I love your Show Jumping images.  What did you take the photographs with?  flickr says I used F2.8 and a 70-200mm lens set to 43mm for this (I also used to cover the local A&P Show).  That was enough to keep the subject sharp, and the background just out of focus enough to make the subject stand out from the background.  If you are using a phone, then try setting it to Portrait mode.  I don't know if it would work with a moving subject like this, but if it does, the the phone will blur the background making the subject pop.  My gut tells me this would be a big ask of the phone, but worth a try.

You can draw in the mask. Or you can use a selection in the mask and fill it with black.

Or you can group your smart object and assign a layer mask to the group and blur the edge there, or or or or ...

If that works for you
have fun
😉

Hey, @Trooper555. Welcome to the Photoshop Community. There are multiple ways to approach this, I'll share one to help you get started. 

You can use the Frame tool to create masks faster and add as many frames on the canvas to add your images within the frame. Head here to get started: https://adobe.ly/3OuZFnQ

Thanks!