Congratulations on finishing your first novel! You've already got a lot further than a lot of people do. Having read through the extract this very much feels like a first draft. You've told the story for yourself and as a writer we need to know and figure out a lot more than what the reader needs. Now you need to edit with the reader in mind.
For example looking at the first chapter, it's all telling and exposition which is not an engaging way for the reader and it keeps them at arm's length instead of getting them immersed. Instead of telling us that she has unusual talent with plants, create a scene where she cares for a plant, or a flower and something happens.
I would suggest using Emma Darwin's writer tool-kit. It covers all the basics of story telling from POVs, effective scenes, psychic distance, show don't tell, etc...
One thing with writing YA is that your story needs to have a strong voice and grab the reader attention straight from the start. I'm not sure how much YA you already read but I would suggest to read critically. For example you can use the 'Look Inside' function on Amazon, look at the opening pages of YA novels and analyse where the writer starts the story, how much into a scene do they start (the advice is enter a scene as late as possible and leave as early as you can), how do they use dialogue to draw the reader in, etc...
Most of the time as writers we over-estimate the amount of information a reader need. Readers like to be intrigued, they want to read to find out things, work things out for themselves. Personally for me, the story starts at chapter 2, but should be shown instead of told, i.e. start with Solstice in a scene, not telling us about, but doing — Digging in the garden, pottering about until she stumbles onto the stone. It's intrigue that moves the story and the reader forwards so remember to raise questions that the reader will want to keep reading to find out.
For example, The Hate U Give by Angie Thomas. The first line, "I shouldn't have come to this party." Simple but straight away as a reader I want to keep reading because I want to find out, what kind of party this is, why didn't they want to come, what kind of teenager doesn't want to go to a party, etc..." and then we delve straight into the party, the writer showing us what's happening.
I hope this helps and good luck with the editing.