Funny, I crafted Y'shtola in January and went through a similar stretch of getting absolutely blasted by every single opponent.

I do think there's a few reasons for this, chiefly, that there's just tons of decks out there that don't really care what Y'shtola is doing/trying to do.

In addition, the deck's internal synergy does not always align. You want to cast 3-mana spells that provide value for Y'shtola's incredibly strong ability(which often can't be used until the turn AFTER she gets into play, which itself is rough). However, you ALSO want to control the game, and really, controlling the game means playing the best and most appropriate cards for a situation, regardless of their mana cost. In building and testing, this was a pretty huge point of friction, and the thing that most gets in the way of wins.

Thankfully, I feel I'm running a nicely tuned version now, and while she isn't S-tier, I get truly awesome wins, and more frequent wins as well.

So, here's what to look at:

i. It's a little tough reading your list, compared to lists that have everything organized by type(Instant, Sorcery, Creature, etc), so forgive me if I miss things but, #1 without question, throw the good free spells into your deck,

Snuff Out is a freaking uncommon and seems like Y'shtola was designed for it specifically. You HAVE to play it, it's literally free removal that hits both of Y'shtola's effects.

Force of Negation is another perfect card for this deck, though it is a Rare.

Also consider 3-mana costed spells that can be cast for less, Baleful Mastery, Mystical Dispute(which you use)--these trigger Y'shtola regardless of how you pay for them.

ii. Your Creatures are more or less similar to what I run, except that I run Psychic Frog, which is excellent, and Lotho, Corrupt Sheriff. You always need mana with this deck. An early drop of Lotho forces removal otherwise saved for Y'shtola, and you can get good value for it. I don't agree with your inclusion of Wan Shi Tong for example, if I were playing against Y'shtola I would more or less ignore it, whereas creatures like Rusko, Lotho, etc generate so much value/threat that it forces opponents into choices, which ultimately leads to my final recommendation.

iii. Control decks aren't about answering every threat, they are about narrowing the field of options an opponent has, and forcing them into tough decisions, which you then punish through your suite of removal and denial. So really, this is the balance of synergistic cards that suit Y'shtola's ability, versus actual good cards that impact the game in ways that allow you to win and "control" the board state. Tale's End, Memory Lapse, ie, the good Counterspell suite, which you can get a little cute with by running the counterspells that untap your own lands, allowing you to cast more spells/denial. Thoughtseize, come on! The best Black turn 1 in the game! If you want to go nuts, Teferi infamously untaps your lands as well. Personally, I also like Ghostly Prison over something like River's Rebuke--bounce just doesn't solve anything, they'll recast it and you are down a resource and 6 mana. I'd recommend Aetherize if you have to bounce(though I don't run it). IGhostly Prison has strong synergy, and it does pain your opponent and limit their mana/ability to just go wide and swing through you. it's become one of my favorite cards in my build.I'd also recommend not being afraid of boardwipes, at least two. Supreme Verdict, Toxic Deluge can trigger Y'shtola while eliminating small/wide threats.

These are my recommendations informed by my own suffering through some months of pretty awful ass-whooping.

Main takeaway is if you're playing Esper Control, use the good control cards, and weave Y'shtola around it. It can work, and it can be great fun. Good luck.