If your distress has some external cause, it is not the thing itself that troubles you, but your own judgement of it - and you can erase this immediately. If it is something in your own attitude that distresses you, no one stops you correcting your view. So too if you are distressed at not achieving some action you think salutary, why not carry on rather than fret? 'But there's an obstacle in the way too solid to move.' No cause for distress, then, since the reason for failure does not lie with you. 'But life is not worth living if I fail in this.' Well then, you must depart this life, as gracious in death as one who does achieve his purpose, and at peace, too, with those who stood in your way.
Don't let life and the external events stick to you. Things cannot actually and literally enter your mind and make you think and feel a certain way. Things don't cause you anxiety; you cause yourself to have anxiety. You intend something to happen and it does not happen. Then find a solution. If there is no solution, what more can you do? Fretting, wringing your hands, worrying, stewing ... all of that kind of activity does not accomplish what you intended to do. So why spend time an effort doing that?
Marcus goes so far as to say you should depart this life if life is not worth living over these things. Indeed, many do take their own life over big and little matters. People take their life because they cannot endure torture and people take their life because they cannot endure bullying. Each of us has to face that reality of whether you can endure what life throws at you or not. If not, be gracious in death as if you had fulfilled your intended desires.
(see also Citadel p. 41, 107, 270)