on the bit during in hand work but not with me!

[Karls Mum]

Hi everyone was after some advice. i have a 15.2h 10 yr old sec d x gelding. ive been trying to get him to work from behind more, through lunging, pessoa and in hand work, which he goes lovely for me and has a good contact and is 'on the bit'. BUT... when i get on him, its impossible! is it me? i was wondering if maybe i am unbalanced and this is upsetting him or if he hasnt developed the muscles yet to carry me and himself? any body else had this problem?
any advice or ideas really appreciated
sam x