There are many aspects of our lives that Allah has decreed. However, we also have a role in making decisions and choosing our fate. As for your case, if you are pursuing something and it’s not happening, try not to be too regretful. It could be the case that it’s not meant for you and Allah has a better plan. So don’t blame yourself. Yes one should work hard and exert reasonable effort. One shouldn’t go to unhealthy extremes to achieve something. Do your part reasonably, leave the rest to Allah and have no regrets.