Whether thoughts are negative or positive is only on aspect in evaluating how helpful or dangerous a thought is to you right now.
It is confusing and hard that the very same thought can propel you forward in one instance and block you in another. ‘Sit with your pain’ is about the ability to tolerate uncomfortable feelings. In one instance it can give you the strength to endure the discomfort that is natural and normal in life. This strengthens you, makes you a more mature human being. However in another situation ‘sit with your pain’ might suggest that the solution lies within you whereas the right action, the action which would solve the situation, would be to talk to whoever you feel has caused these feelings to understand their perspective. In this case ‘sit with your pain’ actually delays the solution.