1. Take time off, like a month or more. [1] Use it to do anything other than coding. Hone a hobby, travel, volunteer, etc. No-one will care about a month or two gap on your resume. 2. You are worth more than you currently think you are. Internalize this, know this, that is key. "I am a sole developer for Mobile and Web platforms at this startup in a very small team" --> is a desirable skill in and of itself. 3. Stop…
But what if they aren't worth more (as a dev, financially) than they think they are? They could be awful, but naively put in a ton of hours. Why do we assume people are worth more? Are we trying to make them feel better by making assumptions and potentially leading them horribly astray? Most people are supremely mediocre and there are just and many horrible devs as their are great ones.
If a little bit of self delusion is what's required to break that cycle, then it's totally worth it.
Not that I think that's what's going on here. If you read between the lines of the OP it's my opinion that the main issues afflicting them are a poor self-image and bad networking skills. They're likely good enough technically but if they don't address the self talk they may never realize that.