Guess it's not easy if you are not used to it but theres a couple of things that might help.
Assuming you know your sight is ok & been recently confirmed you dont need corrective lenses or need them updating. Sorry but it's important to know you are starting off ok.
Shoot in good light, ensure the sights are clear, correct eye relief to rear sight sufficient gap or border if using a square blade, ensure you can focus on front sight. Good contrast between sights & background. Practice for short periods, stop before tiredness or strain kicks in. Take a few deeper than average breaths before taking aim. Dont hold aim too long, concentrate but dont rush. Take time between shots. Establish a time pattern & try to follow it. Rest between shots, look at objects & focus on things at varying distances.
Glasses / shades might help.
Practice, build up stamina over time, don't expect massive changes quickly......patience is key.
I'm lucky I was taught to shoot with both eyes open from the start so it 2nd nature but I can see how it might be less natural to some.
Ps dont think it works with BSA light patterns though
( just kidding)
Edit. Forgot to mention dry practice.......you can pracice without actually needing to fire a pellet, just holding the rifle on aim & replicate the holding/ sighting regime is good practice. Try to replicate lighting conditions & sight picture though.