Well, first and foremost, determine that it is this. Communicate with your leafeon, or observe her behavior. If she seems to act more depressed upon exposure to other eeveelutions (either by seeing them in person or on TV, in movies, or so forth), consider that a confirmation, then proceed with this: reassure her that she made the right decision. Be there for her and give her plenty of affection. Perhaps teach her new techniques to help her better acclimate to her form, and use her in battles so she can see how useful she still is. The more you have her exercise her form and the more you’re there for her, the easier it will be for her to adapt and feel comfortable in her skin.
If, however, her mood doesn’t change—especially if her leaves begin to soften and wilt—then she’s not actually sad or depressed but rather lethargic and sun-starved. Take her outside more often and move her bed to a sunny spot in your home. Increase her water intake and talk to your local Nurse Joy about vitamin supplements for grass-types. If you do all of this, you’ll likely see improvement in her health and mood in no time.
Best of luck, anonymous!