My horse is similarly shaped and I also ride in a treeless. If it's just temporary you could get a cheap treeless (some you can get for around 100-200 and then you'll have it in case you ever need it again) or try to borrow one? Of course in the long term a "cheap" saddle isn't always best, but if it gets her into that one better it might be good.
Anyway, I'm not great with saddle fit, but just make sure you're checking her back for sore spots-where you aren't riding heavily your trainer may be right. Good luck, I know how frustrating it can be!
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.