There's nothing I can say to take that anxiety right away now, but just under a year ago, I was in the same position as you.
All I can say, from my limited experience, is to take you time.
I've not got an easy access site, so I had to think what trades I would pay for and what I could do myself. E.g. even though I've only laid a few brick 40 something years ago, with the support of these guys and some Utubing, I've had a go at the brick and block laying and it's turned out OK. But I can't drive a digger nor get one in my site easily, so I paid for that.
The Building Control Officer at your local council is also a wealth of information and support, and even if you don't go down the B Regs route, they are worth their weight in talking things through with you to get you to that position. If you make an appointment to see one and go armed with drawings, even hand drawn, and rough sizes, and a list of questions written down, they will see that you are trying to do things properly and will give their best advice, which, like in my case, could even be that you don't need BRs.
I tried using open source CAD programs, but didn't get on with them, and so, after a false start, managed to get my head round modelling in Sketchup. That was a huge step and helped me calculate how much concrete for the footings, how many bricks and blocks were needed for the plinth. That helped me draw up a Bill of Quantities and start planning the budget. It will next allow me to calculate the amount of timber and sheathing I'll need for the floor, walls and roof.
There are online mortar calculators so you can work out how many bags of sand and cement you need for the brick and block laying. And so it goes on. All this taking the time over preparation and design has helped me immensely, and of course, the support and advice from these guys is amazing.
The motto of my old Traffic Police department was Festina Lente, which translates as Hasten Slowly.
It's working for me.
PS, I may have got your name wrong, and if so I apologise. But at my age, I get confused with anybody's names these days.