You must believe you are ready for this.
Your belief must be strong enough to keep you going when the publisher / agent you had been pinning your hopes on rejects or ignores you.
Your belief must be strong enough to allow you to keep your head when an agent or publisher does show interest. After all, you are not looking for any publisher or agent for your manuscript; you are looking for the best possible fit.
If you can foster this mindset, a good level of confidence should show up in your submissions documents and all your correspondence with your potential partners.