Editor rejected my paper stating that reviewers had refused to review it long after submission; How can I understand what is wrong with my paper?
The delays you mention are really excessive (note: I feel that way even though I am a mathematician), and your anger is quite deserved for that.

From what I heard from editors I know, it can happen that a paper is sent to successive referees who decline to referee, or worse accept and then don't do the job and do not respond to queries. At some point, the editor have to either do a referee's job themselves, which usually do not happen, or reject the paper. Usually, this happen with papers that are not clearly wrong or uninteresting, but that are difficult to read and do not really interest the tentative referees. In some case, you can have better result by submitting to a different, e.g. more specialized journal.

In your case, what bothers me is that being in a PhD, you have an advisor. What does she think of the paper? If there is no obvious problem in it, did you submit to journal suggested by her? On what ground? Is it possible that your institute, whose PhD committee you says received your work well, has much lower standards than international journals?

In any case, the only effective answer to your question is: seek feedback from a knowledgeable individual. It could be your advisor, but if needed it could also be another senior researcher in your field and institute, or even someone working elsewhere in your field. The closer the better, as it increase the probability of getting the feedback and the probability that it will be honest.

Last, as already mentioned, open access journal can be as toughly selective as subscription journals, and should not be considered easy shots when everything else has failed. I would not recommend submitting to a shady journal, let alone an OA one since shady OA journal often charge several hundred of dollars for publication.