This is a crucial part of growing up. Yes, so of course you used to do all sort of random stuff in college and school, but this is real life. Of course, that was real life too, but you know, you were never considered to be someone fully grown up. Here you will be, so move accordingly.
The first thing that you will have to understand is that rejection is a part of life. Be it a proposal, a refusal, or an offer, rejecting an offer is honest, and you will have to appreciate the honesty of the matter. However, there are persistent people who often try and question the “reason” why it was rejected. They often argue, for long hours, about the rejection, wasting time, raising tempers, and often incurring unnecessary wrath.
Here is something new – don’t argue, accept. Ask a question if you want to, mildly, but don’t expect the answer. It will be much easier to accept rejections that way.