Forget about death and taxes. If there's one thing guaranteed in a screenwriter's life, then it's to expect a whole world of rejection. Not just once or twice, or at the beginning of a career - shyeah, you wish - but on a regular basis.
It happens to everybody ALL THE TIME. If someone hasn't been rejected, they're either lying or they haven't sent anything out yet.
People delivering the rejection will tell you 'it's not personal' but it's always a delicate and sensitive issue that never gets easier the more times you experience it.
Some rejections won't hurt as much as others (a basic 'pass' from a prod co), and some will crush your confidence to the ground (a scathing or tactless review of your script), while some rejections will be unlucky or unfair (someone making the wrong decision on your work).
Bottom line: rejections sucks. No question. But suck it up and keep writing. It's the only way to move forward.