horse help pls!

So theres a Arabian cross mare at my barn who is very well trained on the ground and saddle broke. But she lacks confidence with a rider on her back and won't move away from the gate of the arena. Shes too small for me or my trainer to ride I'm way to heavy for her and so is my trainer. The one person who is a good weight cant work with her often and isnt as expirenced as my trainer and I so we arent sure what to do :/ 

I cant lose weight as im building muscle rn so im actually getting heavier as my muscle builds 

And my trainer would be underweight if she lost some. 

Any way I or her could work with her? Maybe extra padding? Ice baths on her legs after each ride? We only need to work on walking with her and build her confidence so its not like we are running her around. 

Idk we r stumped :( not sure how to help her build confidence again...

Edit as per Jovi. Miss June is about 850 lbs and needs a rider 120lbs or under (im 140) she's around 15 she has 0 previous riding trauma the vet farrier and dentist have checked her up and down and shes clear and we only use positive re enforcment and treat training. She is so wonderful to lead and stands perfect for grooming tacking up and never bites or shows aggression. The only thing is she may be buddy sour but even then we always ride her with either me or my trainer on a horse in the ring with her to attempt to encourage her :/ 

Shes just extremely defensive the second a rider gets on her and we have no clue why as she used to ride fine for year, she has had 2 years off thats the only thing.