Usually shoulders come down to one of two issues - bone spurs in the joint, and/or tears in the rotator cuff tendons.
A simple shoulder X-ray will do much to reveal if any bone spur issues, which will fray the rotator tendons over time
The rotator cuff test is simple - hold your arm horizontal, splitting the distance between front and side (45 degrees), rotate your hand so the thumb is straight down, than slowly lift your arm as high as you can keeping the arm straight....if it hurts in the shoulder, rotator cuff issue..
To strengthen those shouldes Google Thera-Band excercises for shoulders and rotator cuffs, will make a huge difference in shoulder strength.
V slides will reduce pain and increase flex...face the wall about a foot from it - make a blade of your hands and place them against the wall about a foot apart at shoulder height, thumbs toward you - slowly glide your hands up the wall to where you finish fully extended with hands at shoulder width - slowly slide back, and repeat.
Have spent over two years in shoulder rehab after both accident and wear and tear repairs, pins in both, been there ad nauseum
and if the shoulder hurts SH casting, keep your elbow down and hand no higher than your ear...it unloads the shoulder significantly. Buddy has a bad shoulder, he float tubes with a 4 wt switch rod, bottom hand does the work, top hand always below his shoulders...which is how I swing for King's in tidewater