A lot of people on HN grew up in wealthier families, and perhaps never had to work a manual labor job. If the average worker lifts 100 boxes per day, you just have to lift 100 boxes and you know you are doing the work correctly. You deserve to be there.
In manual labor, you never have imposter syndrome if you are doing the work right. Why? It's extremely easy to mentally calculate your benefit to the company and compare it to your fellow workers.
After a day of doing manual work, you always feel refreshed and come home satisfied.
Software on the other hand is much more conceptual, and it is much harder to determine if your input is up to par.
To fix this, you need to figure out goals and milestones with your manager that you can work to reach and exceed - ask your manager what you can do to be in the top 25% of employees, and get a concrete list of features/bugs/etc. on a timeline from your manager for you to complete.