I think you should leave her in her own enclosure next time, she should also be on rats by now. From the picture she looks small for 3 years old, but definitely too big for hopper mice. I'd attempt with a rat pup first. Was she on mice when you got her? I'd also advise against using boiling water at all. Boiling water is far too hot. All you need is hot water from the faucet and leave it in for a couple minutes if it's already thawed...boiling is a little overkill and could potentially hurt your snake.

Don't worry about being nervous, I think it's normal, and I think you'll get over it