Oh and not shame on you for not working on backing up in your first few rides. My boys been in work since he came ott for almost 4 months now and I've only recently started to introduce rein back under saddle. It is far more important to get forward aids first before worrying about rein back, plus some horses will use it as an evasion once they learn they can do it!
Don't focus hugely on rein back, just when you're riding him, maybe do one or two, just casually ask for a couple of steps backwards, then go back into working him again. You really don't want to over do it.
White foot, I very much doubt the bit is hurting him. The OP stated that they are moving their hands around to try and get rein back, this is going to make the horse resist. Quiet hands make a quiet mouth.