I'm going to reply about the bit where you say she's pushing you. It's July. Everyone wants to ride through summer. She's going to be looking for a share horse ASAP and isn't going to want to wait around a month whilst you have a good long think about whether you want a sharer or not. She's most likely coming across pushy because in her eyes, you've had a conversation about sharing the horse and if you want a sharer then she just wants to get on with it. If you're not sure about sharing full stop, I'd tell her so, then she'll stop being "pushy" about starting the arrangement and can also choose to look elsewhere if she wants to, which will do more for your friendship than if you keep stringing her along then decide not to go through with it after all.