Don't worry. You were never able to see both a target that's 100 yards away and a front sight post 30 inches away clearly at the same time. But as we age, our eyes lose the ability to quickly change focus from near to far and back to near.
Even after my recent cataract surgery, I use progressive bifocals while pistol shooting to see the front sight clearly when I have my head level and the target clearly when a bend my head down a few degrees and look through the upper half of the lenses. However, progressive bifocals may not be conducive to rifle shooting when you keep a cheek weld and can't move your head very much.
A set of non-progressive glasses may help you see the front sight, but the targets in the distance will always be blurred. This might be fine if your shooting does not require rapid acquisition of distant targets.
An old pistol shooting drill to emphasize front-sight focus can be adopted to rifle shooting. Set up a plain piece of cardboard 20-25 yards away. Don't put any aiming points on it - no circles, dots, Xs, or anything. Aim at the middle of the target and focus solely on the front sight. (Don't look for the bullets holes.) Watch how the sight moves during and after each shot and keep the front sight completely in focus.
Then next to the plain target set up another cardboard target with circles or squares about 4" in size drawn on with a marker. Aim so your sights are just below the circles or squares (pumpkin on a fence) and keep your front sight in focus. Then change your focus to the target circle or square and then back to the front sight. Press the trigger when you have good sight alignment and the front sight in focus. Follow the front sight throughout the recoil cycle.
Once you have mastered this drill, move the targets out to 50 yards, etc.