I feel like I literally suffer from all of these problems
Easy fixes.
Ez Rule of thumb: Only pick sylv on two lane maps and spider queen never hanamura.
Ez Cookie cutter Talent builds:
https://www.heroesfire.com/hots/talent-calculator/sylvanas#gR8- for haunted mines and boe OR
https://www.heroesfire.com/hots/talent-calculator/sylvanas#hBya for braxis. At 13 will of the forsaken or spell shield can be picked if needed.
Never solo lane, never solo split push, never solo Merc camps... just never solo. Always go to the objective and always push with the objective if you can. Only defend if your team is defending as a team. Sylvanas is a force multiplier, she makes one objective push like two, or a team of 5 push like 10. If she's by herself she multiplies nothing.
Bad wailing arrows? First off never pick mind control, it sucks. The most important thing about wailing arrow you should know is not to cast it first. Wailing arrow is basically always better as either an interrupt or a follow up. First consider if there's something on the enemy team that needs to be interrupted like genji dragon blade or mathiel ult or some channeled ult like lili jugs of a thousand cups or ETC mosh pit. Then just always save your arrow to interrupt. If that stuff isn't there then think of what you have on your team to follow up on. Thrall sunder? Illidan hunt? Your own genji or mathiel is going in? Then follow up by hitting the enemy healer or CC with your silence. Obviously the more people you hit the better, but 9 outta 10 times if you can silence the healer you're doing your job.
Bad positioning? Just stay out of melee range and don't challenge other ranged assassins. Stay behind your tank, clear waves, dagger on CD. Your job is not to play up for extra damage, your job is to punish people who over extend and to always be alive for the objective. That's it.
If you're on a map like spider queen, haunted mines, or boe, and you just don't die the other team is in big trouble.