While walking in the forest still deciding how to properly realise evolve your Eevee to your favourite eeveelutions, but suddenly your Eevee begin glowing, it's a sign of evolution, but that's wasn't that kind of method of evolve her to your favourite one, so she just evolved to Sylveon... Congratulations...? 😕

Honestly you have two choice:

1. Leave your pokemon in your forest and search another one

2. Just surrender and live with your Sylveon somehow... 

But I'll recommend you to not leav your Sylveon, because when you will gonna leave the forest, then something bad will be happen...