help with goin on the bit

[K'n'T]

hi can anybody help me, this is my first horse shes 5 years old n i have decided 2 start dressage next year, she listen 2 my aids quite well, halts square, has good rhythm (so i have been told in a lesson) the only problem is she goes hollow, especially in trot but while in halt she will accept the bit and put her head in to loosen the contact but she wont accept the bit and bridle in trot and canter, i've tried side-reins, draw reins, diff exercise and it doesnt work, any advice?