Self-measurement fails for three reasons, almost every time, and none of them is a lack of skill.
The first is a tape pulled too tight. Every fitting we have ever done starts with the same correction: relax the tape until you can just fit a finger underneath it. A chest or waist measurement taken while sucking in, or taken with the tape cinched for a "cleaner" number, does not describe your body — it describes a version of you that will spend the next several weeks disagreeing with the jacket.
The second is measuring alone when you shouldn't. Shoulder width, back length, and anything that requires the tape to travel across your own back is close to impossible to get right solo, no matter how good the mirror. If you can recruit a second pair of hands for nothing else, recruit them for those three.
The third is treating the first attempt as final. Bodies are not perfectly symmetrical, tape placement drifts by a centimetre without anyone noticing, and a number that looks wrong probably is. That is why our own guided measurement flow checks each value against a real range as you enter it, and flags anything unusual before it ever reaches a cutting table — not to second-guess you, but because a caught mistake at the tape measure is free, and a caught mistake at the fitting is a second fitting.
Twenty unhurried minutes, a tape that isn't fighting you, and a second person for the parts that need one. That is the entire method. Everything else is just doing it carefully.