hey man.
First thing, and I know you probably don't want to hear this, but you really need to listen to waht they are saying. Whether their points about your suitability are valid or not, you owe it to yourself (and to your bank manager) to really investigate whether or not you're cut out for commericial flying. I would highly reccommend taking GAPAN's aptitude tests at Cranwell. Costs about £170 but is a worthwhile investment. Would also give you the chance to chat to some highly experienced pilots, which will give you an even better idea about your suitability.
Next thing to look at is the FTO in question. I assume from the rather limited info you gave us that you're doing a modular course? How well did you do you homework about it? What's their success rate like for exams, IR etc? What's their normal dropout rate? It may be that many people experience similar complaints and move on; or it may be that most people are reasonably satisfied, stay, and complete the course. In this case, these students are most likely experiencing the same conditions as you (in terms of instructor turnover etc). Maybe you need to ask yourself why they might be succeeding where you are not. You may not like what this school has to say, but we all know its a dog eat dog world in commerical flying training. I find it highly doubtful that they would wish to turn you (and your money) away without a well conceived reason; i.e. that you may not have the aptitude, or that you may not suit their school.
Eoin