Working with a supervisor who is not expert in my research field
Can I still be a successful PhD student if I pursue deep natural language processing on my own without getting much help from my supervisor?
The core task of a PhD supervisor is not necessarily to be an expert on the content of your research. Sure, it will help if there is some overlap, but even if you pursue a research direction that is well aligned with your PhD supervisor, it is very likely that you will surpass their knowledge within the first year of your PhD. Key is, that your supervisor can provide you with a tremendous amount of help on how to do research productively. If you get stuck, which strategies are available for overcoming the obstacle? How does one balance reading, programming, writing (everyone prefers some of these tasks over others, but you'll need to overcome your instincts to spend the bulk of your time on your preferred tasks because the other tasks also require time investment) such that publications result? Once the experimental results are in, how does one convert the programming and experimental results into a publishable manuscript? How many citations are enough?
Your questions is best answered when taking two distinct approaches to two halves of the question. &quot;Can I still be a successful PhD student if I pursue deep natural language processing on my own?&quot; Yes. &quot;[...] without getting much help from my supervisor?&quot; It is probably most productive to reframe your thinking about this: you will get much help from your supervisor, just not in the way you imagined.

Should I try finding external collaborator?
This depends a bit on your personal style, and the preferred working style of your supervisor. I am always more than happy to have my PhD students interact with more experts in the field; it's good for the network. In fact, your supervisor may have people in their own professional network that may be helpful in this matter. Consider discussing this with your supervisor: the first paper that the two of you have worked on has been rejected, and you think that the work might benefit from bringing in outside experience. Would your supervisor be open to opening up the collaboration to more NLP researchers?

What should I do?
This is impossible to answer, since it involves you caring for your personal wellbeing in the specific circumstances in which you find yourself. You write that you feel doomed and depressed; you should definitely talk to people, whether those people are mental health professionals, or 'just' friends and family. Asking the question here is a healthy thing to do, but you may also consider finding out whether your university has support groups for PhD students; social events, mailinglists, other ways to interact informally with your peers. Sharing experiences can be very illuminating.