Have you thought about getting her trust prior to putting her in the trailer?
I have just had the same sort of problems with Trinity. Her problem was trusting the places she put her feet when the contrast of the ground changed colour ie the ramps and the matting in the trailer.
She would not put her feet on the trailer for love nor money!! :

When you place a foot on the trailer for her she would then leap in to the trailer and then out the other side but she would avoid the front ramp launching herself out of the trailer
It was to dangerous for both of us to persue this line of training so I thought that if I set up an obstacle course to get her used to trust me when I put her in places she didn't feel comfortable. I could then apply this trust to the trailer environment.
I put a large wooden board on the floor with 2 jump wings either side. I used 2 Jumping poles (one of each wing) on a diagonal so this created "walls" or artifical sides to the wooden board. If I needed to get out the way I could and the poles would fall if knocked.
I then spent about an hour getting Trinity to put 1 foot on the board. I would reward every time she did. SHe then did 2 feet so was rewarded, then she leaped over it so we went back and started again. She ran over it many, many times till I got her to follow me over it at a calm, sensible speed. The next thing was to get her to stand on the wooden board in between the 2 jump pole (ie like she would in a trailer) It took a while but we got there. Every time she tried she was rewarded

I then bought out a big tarpualine for her to walk over.....this took a while but she would calmly walk over it as well as stand on it, all the time being rewarded for the try.
After a 30 min Break, we hitched up the trailer (we had taken everything out so it was a big space). I walked her over the wood, the tarp and then walk up to the ramp of the trailer. A few tentative step then Trinity walked on to the trailer and stopped in the trailer. She was reward again and I showed her the exit and asked her to carefully walk down the ramp.... she did

I kept doing this increasing the time she stood in the trailer (we had a hay net for eating as this helps to calm) Once that was no problem I put the breast bar in, did the same, relax, reward, open the breast bar, out we come.
Then we moved on to the partion being in there but wide, then with it closed to its proper postion.
I have then slowly raised the ramp buy only a little (all the time talking to her) then putting it down and going and rewarding her.
Over time I have little by little increased the height of the ramp to the point were I could close it BUT it was immediately opened and she was rewarded. I have now got to the stage where I can leave the back up but I haven't as yet closed the front ramp (we are still working on this) but I know she is a lot happier with the trailer then she ever was before.
I hope that some of this might help, the biggest thing to remember is that time is what is needed and that it may take a while but it is really worth it.
My next stage of the plan is to close all the doors and drive her round the feild, open the doors and let her out. she's moved but when the doors open again she is still in the same place. We will get to the stage where we will driver out of the feild just up the road, I'll take her out and see if she'll re load again. If not I can always walk her home.

Try not to put to much pressure on the pair of you, you will get their but Baby steps are often what are needed
