If you can't get an interview, it's probably because there's currently far too much competition for every single job. Unless you're putting "I'm really ill!" in big red capital letters on your CV... Probably shouldn't do that
You don't have to put anything on your CV. Application forms will usually ask if you have a disability, and with diabetes it's kind of personal preference whether you say yes or not. Technically a disability is defined as any condition which affects or limits your day to day activities or the work you can do. So diabetes will be a disability for some people but not for others. Sometimes it's useful to tick yes - for example, jobs in the NHS. They have a policy where they will select candidates at random if they get too many applications. But applications from people with a disability are not subject to random selection, so your application would definitely get looked at.
Any employers that take their equal opps policies seriously will separate any identifying details from the relevant bit of the application forms, so the people doing the shortlisting shouldn't be able to see your name, age, ethnicity or disability status during the shortlisting process. Obviously some places are strict about this, such as local authorities and NHS, while some others may be a bit slack. You never know.
Obviously it's illegal for them not to employ you just because you have diabetes. But it's usually hard to prove that's the reason you didn't get the job. Personally I've never seen it as an issue, nobody has ever asked me at interview so I've never volunteered the information. However, once you do get the job, it's very important that your boss and co-workers know. Your employer has to let you go to your appointments, and people will need to know why you're injecting in the office and it helps if your colleagues know how to treat a hypo, just in case you ever need to be helped.
I need to look for a new job as my place is about to go bust. I think I'm going to mention it on any applications for NHS or council jobs, just so I can avoid being randomly selected out. But for small employers that probably don't have to fulfil a quota I don't think I'll bother to mention it until they actually give me a job.
Hope it all picks up for you soon - stay positive and all that!