Tania, re the letter: I probably shouldn't say this either, but what level of forgery are we talking about? Was the old doctor actually addressed on it, or just CC'd? Come to that, was the old doctor's address put on there, or just a name? Because if it's just a name, and it's a separate surgery, and there's no reason for your current GP to contact that doctor, then I would assume that they would completely ignore it. Providing, of course, that the letter doesn't say, "We recommend you contact this lady's former GP, who is Dr X and lives at Y address," but I'm guessing that it doesn't.
If your GP has noticed that your records are incomplete and is going to search for the older ones, you can't stop them. However, if you have a fair amount of contact with your GP, then your records are probably so thick that you could lose most of them when going to a new doctor and still not be spotted!
The "benzos can't do any harm" thing: the problem is that if you develop a tolerance for them, which is very common with benzos, then you can end up back to where you were before in terms of insomnia *and* hooked on an addictive drug which is hellish to come off. I spent a few months on gabapentin, actually for pain though it mainly helped my sleep, and it was an absolute nightmare to come off. I've been off it several months now and my sleep isn't back to usual yet. And I had just had several straight years of fantastic sleep due to the darkness therapy, so I am really irritated about this. I would never have dreamed of trying something so addictive if I had been warned. It's probably still less addictive than a benzodiazepine. I would be really careful there, although I realise that your options in this respect are crap.
The amitriptyline was a reasonable thing to try, don't beat yourself up about it. You may be able to find ways of making your access to meds safer, so that you don't have to worry so much about the risk of doing this kind of thing again. And incidentally, I think most of us have done something similar. I've had a few occasions where I've taken the wrong meds, e.g. co-codamol instead of co-dydramol, and not realised until a long time afterwards. Thankfully it didn't cause anything serious, but it made me horribly aware of what could happen if I did that with, say, a high dose sleeping tablet by mistake.