A manuscript that my supervisors and a collaborator and I have been working on for so long recently got rejected twice by different journals, the main reason being that the findings are considered as "nothing new" and didn't "add anything to the field". However, we do think that our findings contribute to the field and can justify this major reason for rejection. I am thinking about whether we should appeal against the decision. I know it is risky and most likely won't be successful. But I would like to hear what you think about this.Thank you!
You're starting at the wrong end. If four or more reviewers plus two editors have found that your paper has "nothing new", but you think that "our findings contribute to the field", then you haven't made your point well enough in the paper.
The most pragmatic approach to dealing with rejections is to assume that the failure lies with your writing and not ignorant reviewers and editors: If someone doesn't see your point, then you haven't made your point well enough. Any appeal will not change that you haven't written your paper well enough.
So: go back to the drawing board. Read through your introduction and try to understand whether, from the perspective of a reader, you've made it clear enough where the current knowledge has deficits, why we should care about these deficits, and how your paper addresses these deficits. If you make these points clearly and easy to find/understand for a reader, and your papers will get accepted. If you make it difficult for readers to understand the point of the paper, you will get rejections.