I really need help!

You are in a hard situation. This may seem a strange question but is there a reason why they dont believe you ie have you not done them when you should. If there is no reason why then all you can do is keep trying to prove to them that you have done them as for them forcing you to do something it may be worth the pain to prove a point by struggling to do them and when your in pain saying see I told you when it all goes wrong if you can do that if not well I am not sure what to do apart from burst into tears and make a fuss about it until they say fine leave it. I understand a little your situation I am knocked kneed ok as a child it meant there were things I could not do and still can not I can not ride a two wheeled bike or skip. At school most teachers understood but some children did not. Life is hard when you are not like everyone else. It may be their way of coping with what is wrong with you parents do have strange ways of showing they care. I am 36 but if I moan about my knees or hips then mum says well I will do what ever it is and I have to say no I can do it. I assume what you have will only get worse which is a similar situation to me my painful knees will only get more painful as I get older. We both have hard roads ahead. Keep smiling. I just hope some of this helps you.

I understand how you're feeling my little brother has cerebral palsy and he needs to do physiotherapy everyday and he fins it difficult and hard to do. Our parents get on at him for doing his exercises but i have noticed something. They do not offer to help, or encourage. They just yell and tell him to do them. Is that what your parents are doing?
If so you could try engaging them into your exercises too. For example, my brother has to wear gaiters (metal rods rolled in fabric that ties to his legs) to keep his legs straight while he stretches them out. While he's doing that i time him to see if we can beat his time the next day. When he's doing his walking exercisers i count his steps, cheer him on. Ask your parents to do the same.

As for the water, have you tried bottling four litres up and putting it in the fridge, marking each one? That way you can prove to your parents you are drinking the required amount.
They are forcing you to do your exercises because they love you and care for you. That's understandable, but they need to understand that you are in a lot of pain, and they need to take things more slowly. Going too fast with anything can be aggravating in itself, and simple steps can lead a long way in the long run.

I'm sorry to say however that your excuses of it being too difficult and too hard don't cut out for me. Nothing is too difficult or too hard as long as you have the mind set. yes it will be painful. yes it will hurt like hell, but think of the rewards afterwards.
When i was doing ballet, going en pointe was the worst pain i have ever experienced, i'd broken toes, nails, ankles and one my shin which took ages to heal. Did i give up? No, and i can now do it pain free for periods of time without so much as a tremor. get your mind set right, and the world is yours.

I'm sorry that your parents don't believe you. It must be hard enough dealing with every day life without the hassle of your parents nagging. I like the idea of putting water in separate bottles so your parents can check if you are drinking it. I was wondering if it would help if you drew up a schedule for your exercises. You could check off when you completed an exercise. Also, your parents would know when to check to see if you are keeping your schedule. I hope that helps. Just remind yourself you are doing what you can. At least you will be able to feel good about what you are doing. Encouragement is nice and helpful, but sometimes we don't get that and we have to keep to the routine by ourselves. God bless.