For standing, impatience and stubborness are the main enemies.
You havent shot for many years so muscle mass and memory will be shot to hell.
As stated. Little and often and good pace and a routine. If it doesnt feel right, come down. Relax, oxygenate with decent big breaths and restart.
Put the gun down, walk away and relax, and then come back for a few shots then walk away. Its not endurance... that will come but its gradual.
The target is a huge element. Start close and 40mm. As you get more fluid, go to 30 mm at same distance... then 20.... then a reducer.
When your reducer doesnt freak you out, go to 40mm again but push the target back a bit and repeat. Then push back again when that becomes more fluid.
I also did 5 shots each one at different distance and again going 40, 30 .....reducer.
Lots of ways.
Get used to weight SLOWLY. If you cannot shoulder, aim and squeeze of in a fluid movement, come down and start again and that will include putting rifle down, walking away, oxygenating and gently rotating, swing arms shoulders neck etc.
Rhythm is a great helper here.
Its not a race. Nobody wins if you fight the shakes. You get tired and frustrated and you go into a vicious circle.
In a battle of wits I refuse to engage with an unarmed person.
To one shot one kill, you need to seek the S. Kill only comes from Skill