long since相当于long before now(很久以前),这是牛津词典的解释.
因此,I have long since abandoned the notion相当于 I have abandoned the notion long before,意思是“我很久以前就已经抛弃这个观念了”
I have long 只是把应该放在句末的long 提到了abandon之前.
希望对您有所帮助.