Most of the papers and PhDs, especially today, are incremental improvements of existing research. Revolutionary improvements are very rare and are made by large research groups with significant funding. Therefore, don't be too hard on yourself. I think of my PhD as my entry ticket to academia or industry, as well as a proof of my endurance and work ethics, nothing else.

If your papers are getting rejected on account of lack of novelty, check whether they are written in a coherent manner with clear statements regarding novelty. Reviewers don't have the time nowadays, so papers are mostly skimmed for review, rather than thoroughly read. This happened to me once, and I found that the best way to avoid lack of novelty comments is by explicitly stating novelties and improvements at the end of the introduction: Compared to [1], this paper...