So much of Murray’s “mental weakness” comes from the fact that he looks half terrified of hitting a 2nd serve.. never ever in my life seen a top player who gets that tilted when he misses a first serve. Rogie, Novak, Pete, even Rafa just take a breath and get on with it.Mental weakness is a overrated term in Tennis, at this level every sportsman who has already won a slam is mentally tough or else they would not be competing in the 2nd week.
Mental toughness also arises from your confidence in how good your skill is working, Novak has confidence in his return and court coverage, so he can take good serve and return it.... that is how he is tough.
It is like you attend a maths exam and don't know anything, so no matter how tough you are mentally and decide to not give up and fill the paper with philosophy, you will still get 0 marks.... no step marks here and there to get grace marks and score 40% ....
Murray looks like he’s lost the point before he even attempts the 2nd serve. And you’re right, it stems from technical deficiency.