Can you please describe the toy/plug?
I have a jeweled plug as well and it stays in. I even went out shopping with it plugged in
Mine has a thin neck but about 5” circumstance at widest point. It’s the “Back Play Gem” from this website.
Because of how thin the neck is and how fast it gets to wide point, it stays in to the point that you need a little tug to pull it out.

It is a Booty Sparks Gem anal plug from Adam & Eve. Its the small one, 1" diameter with a neck about 1 cm. I place it in all the she'll go. It will feel like its in pretty tight, but if i walk around or bend over.... it pops out.

The one I have is a different material and a little flexible which might be helping.
I had to prectice relaxing or else the muscles inside would push it out.
When you’re about to plug it in, try fully relaxing and when it is plugged, try not to think too much of it and with little practice it should work! I haven’t tried the material your plug is made out of but heard it’s nice! I like the one I have because it’s not heavy, soft yet don’t slide right out. I would think metal is a bit heavier and more slippery so might be good for play and practice to use your muscles there?

Oh great! So good to know all worked! Enjoy