When he goes into the trailer give him kisses and hugs and lots of treats! Make him feel like when he goes in he knows he's safe. If there's a place that he really anjoys (paddock,stall,etc.) then open a side door to the trailer and make him walk through and out the side door into his happy place
All of the below advice is great! I would add: don't be afraid to bribe her either. This worked well on a couple stubborn mares I had. One was genuinely trailer shy, the other was just a smart little turd. I used feed, hay, and treats of all kinds for them. I even bedded the trailer in grass once, which finally got Little Turd in so we could deliver her to her new owners.
If shes truely terrified, dont push her. Just ask for a foot or 2 on the ramp and if she stands calmly ask her to step forward, any forward movement is praiseworthy, even if its barely moving. If shes relaxed with 2 feet on the ramp see if she will put all 4, she can stand sideways on the ramp if thats what it takes. She just has to realise that the trailer really isnt scary. I wouldnt give her any treats or grain or hay, bribery isnt trust its just bribery. Also, if she walks in the trailer, dont just close the door immediatly, as tempting as it can be if you lock her in she may become more scared. Ask for lots of movement in her feet, outside of the trailer. Make the trailer seem safe, and restfull. If she has to work outside the trailer and gets to rest inside the trailer she may be more willing to stand in the trailer.
Just try giving her gentle taps on the butt every 5 minutes and let her sniff the trailer and open all the windows so there is light and so it's not as scary and try giving her treats every time she steps on the trailer and don't pull on her
This is a tricky one, and I was in the same situation myself. If she's very big it might be a good investment to invest in one of the larger boxes. We spent a lot of time just training our girl at home, with everything open and just walking her in and out over and over again. She loaded 9/10 times but there were always days where we had a struggle that must've reminded her of the accident