I understand your thoughts and feelings exactly, because i have been in the same situation in the past. Thinking, what is the point if i will still have another illness hindering me from living life the way i want anyway. What got me down the most was that i could do everything right with my CF care but still end up very sick, and that just made me feel like it wasnt worth it to recover from my eating disorder, because either way i would be in hospital - or that is how i felt. Because even if i recovered from one illness, i would still have the other.
But i was told something very smart by one of my case managers at the eating disorder clinic i went to and that was that 1) I didnt choose either of my illnesses, but i could choose recovery from anorexia/depression/self harm. I cant recover from my Cystic Fibrosis and the only things i can do is take my medication and follow the right routines and live a healthy lifestyle, but i could recover from my eating disorder. I also had to realise that why make my life even harder with 2 (or more ?) illnesses, when i already had one chronic one to deal with. But also to make the connection that my eating disorder was making my CF health care worse which just made me even more sick. But by recovering from my eating disorder my CF care would automaticaly improve somewhat.
One thing i would suggest is going to a therapist or talking to someone about these feelings you feel, because they are understandable. But it can be every helpful to talk to someone who can give you advice and support. But also you have to realise that you can not let your chronic disease control your life or hold you back in life. Even if it might hinder you from doing some things, you can not let that stop you. You can still live a much better life with just that chronic disease rather than struggling with both a physical chronic disease and different mental illnesses. Also think about the fact that you CAN recover from your mental illnesses, even if its tough.
Your life may never be like a movie or you may never have a life the same as your friends because of your disease but that doesnt mean that you can't have a great life anyway and a much happier one when you dont struggle with an eating disorder. Those negative thoughts in your head want to keep you trapped and your eating disorder wants to keep you trapped in the negative spiral and cycle, telling you that things wont ever be better so why bother. But things CAN get better.
Focus on the positives and focus on what you CAN do. I dont know much about your illness, so i dont know how much it holds you back or in how much pain you may be in. But i am sure there are things you can do and things you can experience anyway! But also rememeber that your eating disorder and not eating enough as well as purging and self harming are most likely making your illness worse and leaving you in both mental pain and even more physical pain, so you are just making things worse, when you could be making things better and easier for you by making the changes and choices necessary to recover.
Try to focus on positives. On what you can do instead of what you can't do. Set up goals and dreams which you know are achieveable and also find your reason to recover. Because recovery is always worth it.
Keep fighting and dont give up. I know its awful to have an illness that holds you back, but it will only hold you back as much as you allow it to hold you back. Think of all the amazing achievements people wth different illnesses or handicaps have achieved - they didnt let their disease or illness stop them from achieving their goals, so dont allow yours to do that for you either!