1. More is better. Practice until it hurts. You need repetition and drills to really get the piece into your system.
Consider this thought: There are many ways of learning, including listening, visualization from a first-person or a third person perspective, improvising in the chord/rhythm/style of the passage, and alternating activities or passages, as well as repetition of the same passage in the same way again and again.
2. If it gets really bad you should stop altogether and lose all work on your instrument. Complete rest is the only alternative.
See #1 above.
3. Playing should feel hard for you to know you’re getting something done.
Consider this thought: The body communicates through feelings. If you don’t listen to your child as they say, “I’m having fun”, “I’m hungry”, “I’m upset”, or “My stomach hurts, I’m so hungry”, they will start screaming for your attention.
Your body communicates different messages in different degrees, too, but if you aren’t listening to it, it needs to resort to “screaming” in pain! There could be a lot of helpful messages for you buried underneath the pain.
4. Go it alone. If you have pain, that’s something to keep to yourself because it just means that you need to practice harder to get stronger. Only see a doctor if the pain gets so bad that you can no longer play two weeks before a huge concert.
Athletes have doctors, therapists, trainers, and psychologists on their teams in order to play their best. They don’t expect their bodies to magically feel perfect all the time and then berate themselves for consulting experts when necessary.
You use your body just as finely and constantly as a musician! Have a team of experts in place to keep you at your best and get you back on track before things get out of hand.
5. Pain is a sign of weakness and should be ignored until it goes away.
See #3 above. Also consider that most musicians do not lack the strength to play their instruments. In fact, they are overusing the areas in pain, and the last thing they need is to strength train muscles that are overcompensating for other areas or for poor body mechanics!
Strength may be needed in the large areas of the body for overall body stability and endurance, but the small muscles of the fingers and forearms usually do not need any more work than they already get!