Rashford was desperate to leave Man United in the hope that he could return to his best form, and when Barcelona gave him the opportunity, he felt that it was time for him to shine again. Unfortunately, his hopes didn't quite pan out, and maybe he forgot that the fight at Barcelona was much tougher than at Man United. He was kicked out of Man United because of his attitude and laziness, so if he can't fix that problem, then things could get worse at Barcelona because there are so many great players there, and everyone has to fight for their place.
Going to Barcelona from United wouldn't solve the problem for him because he has failed to work on himself in order to improve his performance. I thought that he must have learned his lesson from what Amorim did to him but he hasn't, because he still show the signs of laziness in him and that's why he's finding it difficult to improve his performance. Maybe, we have already seen the best of Rashford and he has nothing to show us anymore.
It isn't always about learning a lesson. At times it is about pure talent and the ability to bring it on. Rashford is not among the top three offenders in Barcelona. I doubt he will ever be a starter for more than three games in a row unless he must play because the others are injured. Under normal circumstances he will be coming from the bench or play when Flick knows he can give others a rest. There is just no position for him where he would instantly be considered the best among those who are available, simply because he isn't. He has good moments, like Sancho for example, but they can't play on the same level for very long.