I'm an experienced web developer at a very small software development company. We are a startup with 4 months to deliver a prototype, and no project manager. My coworker is significantly older than me, with some web development work, but is not familiar with modern web development. As a result I have been required to teach this developer various web technologies (Typescript, React, etc).
I am having difficulty utilizing feedback from this older developer who is less experienced in the specific work we are doing.
The issue arises whenever I try to explain my code, or discuss new features. My coworker almost always manages to say some variation of the phrase: "I'm sure there's a library that can do that". I would be open to this, but their suggestions often don't make sense to me.
I am not a very confident developer, and so the constant repetition that I am writing code wrong, without any real concrete guidance on what to change, is demotivating me and seriously slowing down my development speed.
How can I productively respond to this criticism?