Remember that you not only have to master the pattern and the cloth, but also the tools, and learning to steer a sewing machine well enough to get those perfect seams takes months of practice - every day! It's a skilled operation.
G&H usually do 3-4 fittings for a suit, but may do more for one of the closer fitting dress uniforms such as the Royal Artillery: >
Yes, but you have been doing this for a couple of months, and NOT as a full time job. You cannot expect to have the same physical skill and facility with the stuff of shirtmaking as one who may have been making the damned things every day for 40 years! Tailors don't make shirts: they send the work out to shirtmakers, the same way they send the buttonholes out to be done (by hand by the best tailors) to buttonhole stitchers who do nothing for 8 hours a day 5 days a week but sew buttonholes. I've been sewing most days of my life for over 40 years, and I don't pretend to have their skills. That's partly what you are paying for with this stuff.
Good move. learn to handle the machine and fabric, to make good straight, strong seams, and then start on the posh frockery. ;)
My first garment project (at the age of seven) was a dirndle style skirt with two seams, a waistband, a zip and a hem. My mum forgot to tell me zips were supposed to be 'difficult', and I rarely have problems with them. The shirt is the same: there really is nothing particularly difficult about making a good shirt, but the skills to make it look professional only come with time and lots of practice. Be kind to yourself: don't rush it, give yourself time, and don't think you are useless if one little thing goes wrong. We all make errors and we all have to do things several times to embed the skills.
Oooh, I dunno... Even the apprenteces have to work on this sort of thing sometimes: >