When it comes to protecting your neck/back/the rest of your body from whiplash or falling, I would suggest talking to your instructor to ask, specifically about how you’re likely to fall, as in, on what surfaces and at what speeds. If your instructor is willing, they would be the best person to learn to fall safely from.
If your instructor can’t/won’t teach you to fall, my advice would be to go to a local jiu-jitsu, judo, or aikido school, and ask them if you can take lessons on learning to fall and roll. Tell them what you’re like to fall/roll on, and what speeds you need to worry about. Most of them should be very open to teaching you, so long as you’re fit enough to train safely, but be aware that this will take a while. If you’re real lucky, you’ll find a teacher who will (eventually) make you train in your riding gear. It’s all well and good to roll/fall perfectly in a uniform, designed for martial arts, but that won’t help if you don’t know how to do so safely in your vest and helmet.
I recently took an aikido class and it took me an hour or so to get rolling backwards close to right (so that I wouldn’t bruise/bump my back, shoulders, or butt as I did so). I’m learning a Korean style right now that also teaches falling and rolls, but I’m not certain enough of my skills to advise you how to fall or roll safely. The best bet, like I said, is to get some training.
Note: if you go with a martial arts school, don’t sign anything right away - ask to visit classes first, see if you like the atmosphere. And, by all means, check out the instructor’s credentials online. The last thing you want is to learn from someone who doesn’t know what they’re doing, and most good instructors will understand and applaud your caution.
