There's lots of good advice here already but one thing I don't see suggested is whether this might be laying the groundwork to let you go... and that might not be related at all to your ability as an engineer/developer.

It's not uncommon for companies to use "Performance Improvement Plans" as a structure for this. You get set targets that are difficult to reach and when you don't reach them, they conclude you're not a good fit. They're awful.

My advice would be to look out for this. If it starts being mentioned, it's time to start looking for work elsewhere. Try not to buy into the idea you're no good, they never fostered the opportunity for you to be excellent.

I'm sorry to read your experience so far. You seem passionate and willing. While it'd be proactive of you to look at other projects it's a red flag to me your manager would be critical that you didn't think to do that... why would you? They should be setting the standard and it's pretty clear from their comms they're not.

Best of luck.