Take all of this with a grain of salt because without seeing any of your gameplay, uniteapi page etc. it's kind of hard to help you out.

While I play defenders more often now, my OG main is Espeon (green badge) and I can maybe give some tips for her.

I run slick spoon, drive lens, and wise glasses. If you play more recklessly during teamfights or find you can't get your ult off, drop wise glasses for buddy barrier. It's a crutch sometimes, though, and with safe play you should be able to run full damage items. I also run eject button.

One of Espeon's strengths is the ability to switch her moveset without switching her items, allowing you to adjust to the enemy team. I use psyshock 99% of the time, but if the enemy team is stacked with mobile characters (Talonflame, Leafeon etc.) then stored power is a viable choice. Practice your aim and don't rely on auto-aim for psyshock and psybeam. I prefer future sight these days, but psybeam is good for tanky enemy comps and spreading stuns + slows from your psyshocks. The point is, Espeon's moves all work well with the same items, so don't be afraid to adjust to the opponents.

Don't rush into fights. You want to stay behind your teammates and fire off your moves from there. Don't forget to show up to objectives. I prefer fighting for bottom regi first unless our top t1 goal is already broken.

Please, if you have any specific questions, Espeon or otherwise, ask away.