10 Balls for Mewtwo - and I didn't catch it for my gf

Simple answer is gold is best. Less simple answer is RNG.

RNG. While you have a high chance of catching it, it's never going to be 100%, even if you had a hundred balls at your disposal.

I usually don't give people sympathy for missing a legendary aiming with great throws...but you hit 9 excellents out of your 10 so that's almost as good as you get so well done on that

However, 10 balls is a bit of a questionable thing:

By my estimation your 10 balls came from 6 base+3 team+1 personal. However, was a further subdivision of your team possible? If yes, you should have done it so you get to do more damage instead of the 1 personal damage ball. Mewtwo can be defeated by 3 or 4 players (level 38+) or 6-7 (level 35-). You could've got 2 or 1 extra damage balls. That could've been all you needed.

It is completly luck, I have silver psychic medal and caught 82% iv mewtwo 15 attack with first ball with golden razz + curve ball only

100% Bona Fide RNG. Maybe very unlucky and like 1% to 2% chance.

If it makes you feel worse, I caught him with a Nice curve Nanab after missing 3 times in a row. I seem to have more luck with Nanab berries than Golden Razz lately

From all I have heard and read, Mewtwo appears to be much harder to catch now that the ex raid passes are more common (as it should be for a legendary). It's all RNG... I've caught a Groudon with the first ball, the last ball, and didn't catch several; number of balls doesn't seem to matter that much (even worse with Kyogre: only have a 25% catch rate, even with great and some excellent throws).

That said, I have little sympathy for alt accounts, and don't think they should get Mewtwo. But I do know someone who catches better legendaries with his alt (25) than his main account (38), not what he wants of course. Justice, I guess.

Did she break up with you?

If so, let's hope you got one as a consolation prize.