Needs consistency. Is dropped whenever he does anything good and mentally, it's like starting from scratch again. I think there is a player there, needs to improve upper body strength a bit and composure but ability and brain wise, I think he is good. In my opinion, he has done more than Rashford this season.
He is typically always looking to support and play others on, or mindful of his defensive responsibilities in the event of turnover ball, at the detriment of being better positioned and actively seeking goalscoring opportunities.
I've noticed over the last month or so both for us and Uruguay the intent to do more of the latter, which wouldn't be a bad thing in and of itself as he always lacked agression and goalscoring instincts in the final third. It's an area for improvement, absolutely, but such intent strikes me as borne in anxiety and unsurprisingly isn't translating to results. There is probably an element of what you are saying (frustration, every chance being make or break) in that.
Both that and the fact all young players spend tonnes of time on social media/websites like this one reading and being affected by what they read. Imagine reading this kneejerk threads after each and every performance...
I met the coach that just got Uruguay to win the U20 World Cup and he placed great emphasis on managing that. He had someone in his staff employed to track online activity linked to all players so that they could proactively address any potential effects. I know every team has psychologists, etc, but was surprised really at how listening to him it seemed to be 60% of his job rather than yapping about tactics, following his instructions and whatnot.