Got my first rejection letter in 3 years" - that actually tells me you've been on a pretty good streak. Rejections sting no matter what, but maybe look at it like this: if you hadn't been putting yourself out there enough to get rejected, you wouldn't be growing either. Take a day to be annoyed, then brush it off and keep going because that's clearly been working for you.
Was the rejection for something you really wanted, or just one of those applications you sent out hoping for the best? That makes a big difference on how you process it.