Throughout your career, you will have a lot of manuscripts rejected. It’s something that happens to everyone who submits to academic journals. The most renowned professors in your field will still regularly have manuscripts turned down. I’m sure that your supervisor would be able to tell you about recent rejections that they’ve received. It can be difficult to avoid feeling disappointed about not being published in your first-choice journal, but it’s important to remember that a rejection is not a failure: it doesn’t mean that your paper isn’t good. It just means that it’s not quite right for that journal at that time. Keep in mind that editors make curatorial decisions as well as quality decisions.
When you receive a rejection, try to take the constructive parts of the reviews and editor comments and leave the rest. If your paper has gone through the peer review process, you should get at least a couple of suggestions for how to improve it. You might also get ideas for other potential manuscripts or avenues of inquiry for future research. At the absolute minimum, you’ll have gained some experience of the publishing process and hopefully that will make things easier the next time you submit a manuscript somewhere for consideration.
The best thing that you can do after a rejection is submit your manuscript to another journal. A different editor and set of reviewers may see something that the first group missed or your paper may be a better fit for what that journal is looking for at that time.
Before submitting your manuscript somewhere else, read through your decision letter and make any changes that you agree with. This should improve your paper and give you a better chance of being accepted elsewhere. If your paper is on a relatively niche subject, this is very important. The narrower your field, the more likely it is that the same people will be asked to review your paper. If they can see that it’s the same manuscript that they assessed previously, they may well send the same review.
Finally, before resubmitting, you should revise your cover letter. At the very least, read over it to make sure that you’ve removed any references to the first journal you submitted to. You can find advice about writing a cover letter here.