Honestly... Right now I don't give a **** how you can't afford to buy her a bale of hay, but you can buy a new computer.
That's absolutely no excuse whatsoever for you to DRAG your horse behind a lorry. If I was in your area I would have had the SPCA out to you a long time a go.
Harsh but real.
Normally I'm not one to jump into something like this. But you drug your horse behind a truck and thought nothing of it?! It's one thing to lead them while in/on some type or vehicle, many of us have done that, but to drag her across the ground and not worry if she's hurt her neck is unbelievable. I hate to join the bandwagon but you should NOT have a horse.
If your horse can not even be lead around on a leadline yet, and you had to pull her behind a truck...and you can't afford the correct feeds, water, or or health care for this mare...I'm going to have to agree with everyone else. You need to get rid of her.
I currently have a 21 month old who I've been working with since late December who was severely neglected to the point of abuse, and who has suffered extreme trauma just like your horse. I had her leading like an old school horse within 2 months and she already hard ties, picks up and holds her feet, bathes, leads, yields to pressure, backs up on command, loads into trailers, and isn't pushy about her meals. It is not hard to teach a two year old to lead well enough to bring it in from a dangerous situation...and its inexcusable to not have a halter that fits her. Sell her and be done with it. Maybe you can find a nice place to get riding lessons if you still want to be around horses.