Oh dear good old float training. I have been laughing every time I drive past a property this week. They're got the float in the yard and feeding the horse in it. I drive past in the mornings and at night just as the horses are being fed. And every day this week, the horse has walked in, grabbed a mouthful then run straight back out :P Generally what happens when you try to float 'train' with food.
My biggest question first. Are you trying to tie her into the float before you shut the back? And do you have breach gates on the back of your float???? If you are trying to tie her in before the back is closed, you are just begging for a horse with a broken neck, or at the very least one who pulls off ties every single time. DON'T DO IT!
As for breaching gates. I will NEVER float a horse without them. Far too dangerous. Even chains are dangerous, the horse can barely feel them behind them, and with a rug on the pressure is almost non-existant. I've seen 3 horses now flip over backwards from hitting the chain.
Horses learn via pressure and release/discomfort and comfort. Whatever you want to call it, same principal applies.
Make life outside the float uncomfortable, and life inside the float comfortable. Putting the float in her yard and feeding her in it gives comfort both in and out of the float so there is really no reward for her to stay in there.
I would do a lot of ground work with her before you even introduce the float. Take a dressage whip with you. Ask her to step forward with a slight pull on the halter. If she doesn't respond, tap her on the backside until she goes forward, then release all pressure. If she doesn't move from a single short tap, build up a tapping rhythm, so just a constant pressure (tapping) on her hindquarters until she steps forward. Make sure, that AS SOON as she steps forward, release ALL pressure. If you continue tapping once she's stepped forward there is no release of pressure and she has learnt nothing from the exercise.
After you have achieved forward, make sure you can back her safely. She needs to step back away from you from a light pressure of the halter.
Then you can bring the float into the equation. Take the divider out (please don't float train with a single float by the way) and open the front door so it's nice and airy and she can see light at the other end. ALWAYS have the float attached to a vehicle!!!!!! I can't stress this enough!
Face her up to the float, standing at her shoulder with the dressage whip facing her hindquarters. Ask her to walk on (you have to be assertive and confident here. Treat her like she's a horse that self loads.). If she doesn't walk forward, then tap her, continue tapping like I said above to make life uncomfortable until she steps forward, then take off the pressure. If she tries to go backwards, don't pull on her head, just increase the intensity of tapping until she stops, release the pressure, then ask for forward again. If she spins, DO NOT let her face away from the float. This is a release for her and she has won the battle - making it even harder for you next time round. Just keep tapping until she goes forward.
Continue until she works out that when she moves towards the float, she is comfortable and safe, and when she backs away she is uncomfortable and under pressure. She'll start to figure out that the float is a good place to be!
Once you get her in there, don't just tie her up and take her for a drive. When you float train, don't do it with the aim of getting her to a show or something, do it at least a month in advance. When I float train I don't take them for a drive for at least 4 days depending on how they're going. They have to be totally comfortable in there.
So back to the point, once she's in there, rub the whip over her hindquarters so she knows it's there, but it's not pressuring her. Talk to her, give her a scratch. Let her stand for about 20 seconds, then ask her to back out. If she flies out backwards, go back to the drawing board, higher pressure tapping until she walks back up to the float.
I would put her in and out about 3 times, each time increasing the time she has to stand in the float to about 40 seconds to 1 minute. Don't tie her, just hold her. Don't use food! The next day, same thing. She'll fight to start with, but the fight shouldn't be as strong and then increase time in the float to up to 2 minutes depending on how she's going.
Day 3 is when I'll start shutting the back on her. I never shut the gates on a horse that is nervous in there and likely to panic with the pressure behind him. It is too easy for them to hit the back and flip over (WIDE breach gates are the best thing to prevent this from occurring). Gently close the gate behind her (get another person is probably easier) and keep rubbing her hindquarters with the whip so she knows that if she tries to back out she will receive pressure from the whip.
Let her stand for a minute or so, then open the back and ask her to back out. Repeat it a couple of times until she is confidently walking in and out and not worried about the back being closed on her.
Once she's confident at this point you can start taking her on short drives around the block. Coming home, getting her out, then putting her straight back in. Don't ever just walk the horse and and put them away straight away, you want to be sure they'll go back in.
Then start taking her for trips that are walking distance from home, so once you unload her if she absolutely WILL NOT re-load you can always walk her home (in this instance it would only be because the area is too dangerous to be training is she spins around. So pick an enclosed safe area to go.
Hope this has helped you a little. If you struggle, just get a trainer. Once a horse has a bad floating experience or pulled the wool over your eyes it is so hard to re-train them so I would HIGHLY recommend going to a trainer.