Hi Fay,
This is not my strongest skill in PS. I can see what you have probably been doing. When you add a layer mask you just get a circle (which is a brush) and cannot clearly see the edges of what you are masking.
There is a 50 second video in PS when you select a layer mask and this shows how you can use 'Select/Subject' to refine what you want to mask.
Here's one I did:

and the end result...

There's lots more to it than this as I found out when I went into the 'Refine' area just above the layers panel.
Good luck!