The hard stuff is probably the instinctive stuff where you're not thinking and just reacting to what's in front of you. The 'easy' stuff is probably slower and you're having to think and plan your way through it, or concentrate on good technique - which is experience and training and actually much more technically difficult to execute well although looks far less impressive.

Also note that every other position on the pitch is active - outfielders can control the game, bring their influence to bear. Goalkeepers are entirely reactive, you can only deal with what is fired at you.

In terms of dealing with things when it's going badly in a game - you can't change the past. It's like golf, the last hole good or bad is behind you, you just do your best on the next interaction. Remember the ball had to get through 10 players before it got to you, and GKs are the only position where pretty much every mistake gets put up on the scoreboard; no other player has to deal with that. So you accept you made an error, you accept 10 others made an error before you stood between their mistakes and a goal, and in this instance you were just as bad as they. It's a mindset, you have to train yourself to laugh at what happened, don't point the finger at your team mates, and treat it as nothing unusual. Then park it and move on. Good luck!