When you are in pain for a long time, you are often told to accept this. It seems as if by this someone means that you can't do anything about your pain anymore. As if there is nothing left to change, and you will always keep this pain. 
Personally, I find this form of accepting nonsense. It is a shame not to do anything about your complaints anymore. There is also so much evidence that pain can improve, that it is also wrong to think that no progress is possible. In fact, thinking that it will never get better can cause a negative spiral that exacerbates your pain! 

What to do with acceptance then?

A well-known saying of Fransiscus of Asisi goes as follows:

Give me the calmness to accept what I can't change,
the courage to change what I can change,
and the wisdom to know the difference.

Acceptance is certainly important when it comes to pain. But there is a difference between what you accept, and where you strive for improvement. 

1. Accept that this is your situation now. You can't change the fact that you are in pain, that's just the way it is.
2. Let go of what has been. Maybe you have tried a lot of treatments for your pain, with varying results. If that impedes your progress now, it is good to let go of it.
3. Do not compare yourself with the past, or with others. Look at each moment to see how you can make progress. Appreciate your efforts per day and look at your progress over longer periods of time.
4. Don't stare blindly at your pain. Pain can be reduced by applying new behavior, but don't try to emphasize it. Before you know it, you are always trying to evaluate your pain and it makes you unhappy again. The more attention you pay directly to pain, the more your brain is concerned about it. 

So expect and strive for progress without being blind to pain as a direct result. Because change is possible.