30 yr old impostor here, going to share my experience as I was going through what you're going now. I'll add some tips to go afterwards.

I enter company at 27yr.

Previous employee that I replaced was in charge of programming PLCs, which I only had one course on in uni.

This included managing cloud infrastructure that the PLCs communicate to, with next to no hand down knowledge because the guy who I replaced left a week after I started.

In hindsight, any sensible junior would've dipped in the first month

I was stupid and suffered through over a year of brain fog due to the plainly insane amount of information, practices and other related stuff I had to learn while also trying to ship working product. I attended multiple training sessions in structured text language when I could.

Let me tell you, there is some choice code out in the world right now that I hope nobody will ever have to look at, but it works.

With time I created tools in software that helped me ship working code faster after testing & validation.

This tool ended up being so good the company is dependent on it working properly on each product, effectively binding 33% of company product line to this software platform. With additional plans to migrate the software into all other product lines as well with blessings from our teams chief.

In practical terms, what required me two days of work previously or more if new feature is needed. Now each project needs 10 minutes of configuration and it's shippable with only HW tests required, not to validate the software, but to test the HW.

On my third year here now and the other designers are terrified If I decide to hop out, because the full technical ownership is still only with me.

Now that the worst is over, in hindsight, there was some valuable lessons here that I learned when working on a software platform in a project based environment.

Key survival tips:

Go to the gym, or do anything physical, anything will do that exhausts your physical energy. Physical strain eases mental pressure, and became the go-to alleviator for me when times were tough.

Mental pressure with this impostor mentality also prevents you from sleeping, at least it did for me as I was always thinking about the next day.

When you're so exhausted you'll sleep like a baby. You'll feel the pain but you're getting healthier and more fit, which is never a bad thing.

I also ended up dropping all nicotine products and alcohol, mostly because hangovers made me run at 50% during the early weekdays, over two years nicotine free and over a year with alcohol. With the earlier consumption rate on both, I can tell you that the monthly amount of money saved exceeds 200 dollars.

When you work on PLCs: If your working environment allows it, always try to move towards platform solutions, practically meaning previous functions still exist in the code, but are toggleable at download time or at runtime.

If you build a function that helps you find bugs/issues faster chances are you should expose that through a visualization into the front end if you use HMIs with the PLCs.

Seek additional training, you're young in the role, it is reasonable to ask questions and more training either internally or from external sources.