Question

If C"V someone commits suicide because he is in terrible physical pain for a very long time and just can't cope with it anymore;

is he held responsible for it, and is he punished for it in Shamayim?

Thank you.

It depends. 

It is possible for a person to be driven to do a sin by circumstances so strong that he cannot resist. In such a case, he is not held responsible because he did not sin by choice. It is considered as if he was forced.

Whether any given person who commits suicide has been driven to do so beyond the point of his being able to refrain from doing so, I am unable to say. But theoretically, it is possible.