As previously mentioned, . . . practice is the key to performing under stress.
No pun intended, . . . but that fact cannot be stressed enough.
First thing: play around with your setup and see what "works" for you. For example, I use a Beretta semi auto, . . . so I can have a 5 shot sleeve on my fore end, . . . your Mossberg is a trombone so that won't work for you.
Most often in pictures, . . . you'll see a stock sleeve with shells in it, . . . bases pointed up. For me that is wrong, . . . it is ergonomically a whole lot better for me if the bases are pointed down, . . . YMMV.
I do not have a sling on my shotgun when it is in HD mode, . . . but for rabbits afield, . . . it goes back on.
Because it is a slide action, . . . you can take some fired shells, . . . load em up like dummies with plaster of paris or some such stuff, . . . and simply practice loading, shucking, reloading, shucking, etc.
Try different ways of loading, . . . try both hands, . . . different positions, . . . there is no "Perfect" way, . . . just the one that works for you.
I can load my shotgun in the dark, . . . looking out the window, . . . listening for other noises, . . . but only because I have practiced it so that it has become the type of muscle memory like tying shoes, . . . you just do it and don't think about it.
One final point: develop "your" plan. Do it the same way every time. DO NOT try this today, that tomorrow, something els on Friday, . . . you will not function when the time comes that you need to. You will stop to try to reason out what you need to do, . . . an by then it just may be too late. Get a plan together, . . . practice it, . . . learn it, . . . and don't chang it unless you get some REALLY good reason to do it.
May God bless,
If you can breathe, . . . thank God!
If you can read, . . . thank a teacher!
If you are reading this in English, . . . thank a Veteran!