If you don't get the tap, sweep, or pass, 9 times out of 10, you should let it go, reset, and move on as you have done something wrong.
I have been guilty of this through out the years of training. I'll set-up and go for a new sub, sweep, or pass but not fully get it set, trapped, or otherwise locked in correctly. Rather than letting it go, I continue to try and force the sub, sweep or pass. The problem here is that your opponent has already reacted to the attempt, adjusted their base, and will be in the process of countering and attacking while you are still hoping to hit a lost opportunity.
By continuing to grasp at straws you will get countered and quickly end up in a defensive position or may even open yourself up to a submission attempt due to over-reaching, which is a consiquence of continuing to go for something that is not there.
The guys on the mat are your training partners and teammates, NOT submission notches in your belt. So, getting a tap to a cross-collar choke that is set across your teammates nose is not a submission...you're just a dick.
You will frequently be in positions that your instructor hasn't talked about and even if you do recoginze what is going on, you forget what you are supposed to do because you panic.
Two things I can think of are Hygiene/ wash your damn gi, and remember that there is no "winning" at rolling. You are practicing on another person techniques that could potentially harm them and they are doing the same with you, with the understanding that you aren't going to be a douche. I got into side control with one of the other new guys, and after being at a standstill for a minute, asked him if he was taught how to escape side control yet. He said no. We agreed that there was no point in hanging out there and just started over.
When rolling with a higher belt feel free to ask questions, especially when you don't understand what happened. I mean don't spend the whole timing talking off their ear, but feel free to ask about specific things you are working on.
When working on staying heavy ask them if you are heavier like this or like that.
Personally, I tell brand new rollers relax, focus on your breathing. Keep it simple, no matter what position you're in, work to better your position. If you're in a dominant position such as mount, or if you're in guard, just work to keep it. Don't get any more complicated than that.
I still focus on my breathing, it keeps me calm and relaxed.
I just started BJJ last week and I'm finding a lot of these comments very helpful.