To communicate effectively that someone is moving too fast and you want to slow down, it's important to know exactly how you feel rather than just asking them to pump the brakes. Really evaluate whether or not this is something that you may be ready for in the future, even if it's something you're not ready for right now. That should really help you decide how to effectively communicate what you're feeling. Once you're clear on your intentions, it's just important to communicate really transparently, but also really gently.