As someone who has dealt with a work-related overuse injury (tendonitis) myself, I would urge you to take care of your body. If you're experiencing pain, then your body is trying to tell you something. Taking painkillers might make the pain go away, but it does nothing for the underlying cause.
Yes, you should expect to develop endurance over time, but you need to build it up gradually and listen to what your body is telling you, otherwise you are going to do damage. If you feel sore, take a break, even just for 5 minutes.
You mention having a sore shoulder, and also that you keep your (plucking hand) elbow up while playing. Is the sore shoulder on your plucking hand side as well? These two things may be related, and they may be related to the pain in your fingers as well. If you are constantly holding your elbow up, you may well be keeping your neck and shoulder in a constant state of tension.
While undergoing rehabilitation for the tendonitis in my arms, I learned that tension in my neck and upper back was responsible for pain that I felt in my elbow, forearm, and wrist. By increasing the flexibility and strength in my upper back, I was able to relieve the tension in my neck, which caused the pain and stiffness in the rest of my arm to go away.
The main thing to remember is that if the muscles that you are using are relaxed, then they will be at a vastly reduced risk of injury. Strong, relaxed, flexible muscles will do the job much better than weak, tense, cramped muscles. Any tension or pain is a warning sign telling you that something is not right.
