What/who do you think might be an obstacle for Macbeth and Lady Macbeth going forever?

I think they were their own obstacle. Macbeth isn't the same after he kills King Duncan. He is so full of anxiety and paranoia that it is impossible for him to ever go back to the way he was, 'I am in blood / Stepped in so far, that, should I wade no more, / Returning were as tedious as go o'er' (III. 4.136–8). Lady Macbeth is in a constant state of damage control. She is trying to appease Macbeth's increasingly fragile mental condition. The two of them were destined for self-destruction.