Honestly, 300 rejections seems like a paltry amount to me at this point. I've been sending work out for close to 10 years, and that only averages to 30 rejections a year. Granted, that count only tallies magazine rejections. In the beginning I didn't count book rejections because publishing a book was not even on my radar. I was only thinking in terms of the lofty goal of publishing a story or a poem in a tiny lit mag. I could go back and count up the book rejections and job rejections and panel rejections and residency rejections that should be included, too, and maybe that number would feel truer or heftier, but I'm too lazy to do such a thing. The feeling I have right now at crossing the threshold of another hundred is: I probably could have tried harder. But also: I'm a pretty vocal proponent of trying to fit in real life with the writing life, and I feel ok that I'm where I'm at, and with all of the living that has gone on around those rejections.
Rejection 300 was from One Story, and from what I can glean, it was an encouragement. "We liked this. Send us more." That seems like a nice one with which to hit 300.
A friend of mine's book just rang in at #19 on the NY Times Bestseller List and he posted the following from Gary Shteyngart that really is a beautiful sentiment to keep in mind always: "If you’re crazy enough to still write literary fiction, you have to worship your readers, not the other way around. Today I see each of my readers as a gilded unicorn with a cronut hanging off her horn. I sign my books, take a selfie with the unicorn, and go back to my hotel room teary-eyed, because I know that someone in this universe still has time for what I do."