How do I learn to recognise what is worth publishing
Broadly, ask yourself these two questions:


Is it new?
Is it something that people in your field would want to read?


The first question is very important and sometimes disheartening. If it's not new, it doesn't matter how interesting your result is, how difficult it was to obtain, or how well you present it. You say that you have difficulty feeling like your work is new, but (unlike whether or not your work is "good") this is something that you can and should unambiguously determine by reading the existing literature. Note that "new" doesn't have to mean "earthshaking". It can (and usually does) mean more modest novelty.

The second question helps you distinguish between many possible new things to try to publish. If the paper you're working on already existed, would you and your colleagues want to read it?  You can use the degree to which the answer is "yes" to decide whether or not you should spend your time on one publication as opposed to another.

To answer either question, you need to read a lot of the existing literature, and get to know the academic community you work in. So yes, experience is important. Your advisors are there to help you when you are starting out.