It sounds like you need to take on a business partner and share the risk, stress, pressure etc.
A lot easier said than done as finding the right business partner is a minefield in its own right. They don't have to be an equal partner if you are worried about control issues and want to keep a firm hand on direction. Maybe someone that is more interested in the administration side, man management which means they don't have to be necessarily in the plumbing field. This then gives you a wider net to think of possible suitable partners.
Can't be any fun dealing with all the stress and workload yourself, and certainly not good for you. Must have a big black book of local business contacts you can work through, or maybe even approach a small but established firm who may be glad of a lot more work and make a proposition to join them as a partner?
It is odd how business tend to go in unexpected directions due to outside forces or based on a work for particular clients. I think it's this aspect I love, you may think you know your 5 year strategy, but in reality it rarely works out how you thought it would!

Had to make a video conference call the other day and I needed a gadget to make it all work. So thought I'd pop into Maplins our resident local techie and gadget shop on the way to work.