How do you stay calm and collected when you're feeling a tad overwhelmed?

I would love some of your semi-trailer right now.

I also find that it's either a feast or a famine and try to stay calm. I have regular breaks, just to get up and stretch so I don't get too stiff from typing and sitting.

I will watch some TV or read and other times I will meditate.

Try this. It worked with my officemates when a ton of "excrement" gets dumped on our department.

Just ask: "How do you eat an elephant?"
.
.
.
Answer: "One bite at a time..." And just continue with your work. Do what you reasonably can NOW, and it'll be done, "one bite at a time."

Oddly, that's one thought pattern that'd get you through the NAVY SEALS' Hell Week. The ones who reach their breaking point and quit are those who thought of the terrible training they'd have to get through... those who SURVIVED are those who concentrated on the task of the moment, no matter how exhausted or tired they were. (Source: "Resilience" by Eric Greitens)

I'm sure as a business owner you feel that you can't grumble at having 'too much' work on as I'm sure there's no such thing when you're self employed!

I agree completely though that sometimes that overwhelming feeling in the pit of your stomach sets in and it makes it hard to get past what's in front of your computer screen.

I know this feeling well, and while ultimately, it's a GOOD thing for your business, self-care is still the most important factor.

If you don't take steps to combat being regularly overwhelmed you end up in a vicious cycle consisting of stress/anxiety/nerves which brings about a whole host of health problems. This will only be counter-productive as far as crossing things off of your to-do list goes.

I would suggest the following in that situation:

Go for a walk.
I know how hard this is to do when you have a pile of work in front of you, however for me personally, taking a break from the screen and re-grouping ALWAYS works wonders. I can spend an hour looking at something not being able to string a sentence together, or I find that a ten-minute break can lead to 50 minutes of productivity. It's a better outcome even though it doesn't feel like a plausible option at the time.

Getting outside in nature is always refreshing too and I don't think you can beat it as a stimulation for the grey matter.

Meditate.
Again, a ten-minute breathing break always works wonders for me. Meditation can be as simple as taking yourself away from your desk and carrying out some simple breathing exercises for ten minutes. I find that when I do this, I clear my head of all of the stuff that doesn't need to be in there which allows me to focus on my task in hand.

Make a list.
I have lists upon lists of things that I need to do. I have an overall to-do list (so I don't forget anything) a 'today' list which consists of everything that I need to do today (obviously!) and I have a priority list of things that are urgent, broken down and highlighted depending on the order i need to complete them in.

Apart from the satisfaction you get as things are crossed off, I am also a very visual person and this helps me keep things in perspective and even when I FEEL overwhelmed, my list reminds me that it doesn't necessarily need to BE that way.
I often find that if I have a list with ten things on, by the time I cross even one or two off I already start to feel a bit better.

Hire an assistant.
I know you said you are ramping up business with the view to taking on new staff (congratulations by the way!) but you can hire Virtual Assistants through sites such as Freelancer and UpWork. The beauty of these sites is that you can palm off your simple admin tasks without having to employ anybody long-term. The downside of course is that you can sometimes spend longer explaining a task to somebody than it would take you to do it yourself, however if you can find a little treasure in the available resource, this will take a great weight off of you moving forward.

Put it into perspective.
Finally, I'd try to remind yourself of the bigger picture. Create a vision board to put near your desk and fill it with all of the things that your scaled up business is creating for you. A new home, travel plans, financial freedom, add some inspirational quotes too...
Every time you feel overwhelmed, have a look at the board and remind yourself exactly why you're working so hard at the moment - this will be sure to take some of the panic away and give you the extra push you need to keep going.

Good luck with it all, sounds like a very exciting, if a little stressful, time for you