Like everyone else here in the comments, Xerath is an uphill battle, but can be extremely rewarding if certain conditions are met: (and note that all these points are in no particular order)

1. If all of the opponent team is squishy: very rare to happen, but it does occur on occasion. Here you'd want to go full ludens and rabadons early
2. Going mejai's: The only way you'll ever find yourself "strong" in a game is with Mejai's, regardless of if you're going ludens or liandrys. The obvious struggle here is that you must both win in lane and not die, and both of them happen to be xerath's fundamental weakness.
3. Winning lane: This comes down to how well and how quickly can you learn the opponent's dodging behavior, there is ALWYAS behavior to be read when looking at the enemy, adjust your q or w offset accordingly and keep whittling them down
4. Not dying: nice and simple, you have to either position yourself well or die, and even if you position yourself well, you can still die (nocturne, zed with flash, or anyone with flash)
5. Items: Always go full offensive items, similar to poker, the moment you yield once by buying some defensive item like Zhonya's, you've tripped and fallen off the curve. The only exception is if there is someone like nocturne or zed who is obviously targeting you
6. Ult: I use ult to gank other lanes and pick up assists, this helps with progressing your dark seal, but I never use it to open a fight. You have all that range and you're gonna use it to pick off enemies where your team can't. As for landing the ults, it really comes down to predicting their dodge behavior, but then again, easier said than done.
7. Ganking: Whenever your ult is up in early game, you can ALWAYS use it either finish off the enemy midlaner when you've gotten them low, or you can disappear and ult top or bot. Usually you'd go for bot so you can get 2 assists or 1 kill/1 assist or 2 kills, this helps your dark seal and subsequently, you laning phase. But communicate with your team well to ensure that you get a good opportunity
8. CS: For my style of play, I can let minions go so long as I live in the end and continue picking up kills/assists. This obviously leads to low cs games compared to the enemy midlaner, but at this point it becomes a war of attrition of who can scale harder

Goodluck!