A&S 15-1 is in full swing by now. I wish all candidates the best of luck. If you are still in the program, continue to give it all you've got each day. Don't try to be or do what you "think" the instructors are looking for. Just be yourself, and do what you think are the right things.
I want to share a little story with you. Last year, a young Marine attended A&S. He made it all the way to the end, but wasn't selected. When he came back, he was extremely disappointed. He disappeared for a couple of days, and I had zero contact with him. Attempts to reach him were fruitless, and usually ended with me leaving a voicemail. I was getting pretty concerned for his safety, and was on the verge of notifying the command. He finally called me back, and I ordered him to report to me immediately. He had not had a haircut in weeks and hadn't shaved in about 3-4 days. Although I was glad he had safely returned, I completely lost it on him. I had never treated him with anything other than the utmost in professionalism, and I felt my trust had been violated. I got him straightened out, and he eventually returned to his parent unit. Months went by and to my surprise, I received an email from him. He wanted to thank me for my patience and mentorship, and also to apologize for his actions during that time. The Marine will remain nameless, but I told him that his apology showed growth and maturity. I wished him success in his future endeavors.
The moral of the story? If you return from selection without a certificate, don't get angry at the instructors. They had nothing to do with you making it or not making it. It was all based on your own actions and your combined performance (physical/mental). Secondly, do not "drop your ruck" because you don't give a crap anymore. Pick yourself up, and take solace in the fact that you gave it everything you had. (You did, didn't you?). Third, take what you learn about yourself and apply it to the rest of your life. Better yet, share some of what you've learned (You know what you can talk about and what you can't) with other Marines to help make them and your entire unit better.