Emily,
Put simply, you are suffering terribly due to your depression. It sounds very much like clinical depression, which is deep-rooted & is contributing to your wreckless, self-harming behaviour. You don't care about yourself BECAUSE you are clinically depressed. It doesn't matter what caused you to become so mentally unwell at this point in time. Whether that be your diabetes, or not. The important issue right now is that due to your depression, your thought processes are altered & you are unable to recognise the consequences of your actions. This is an acutely dangerous situation. I am telling you that as fact honey. You must seek help.
I appreciate that being a patient on the very medical ward you work is awkward, and that you do not want your colleagues to know what you are doing in respect of your insulin. Nor how you are feeling. But, if you carry on like this, you giving yourself higher than necessary amounts of insulin will eventually lead to your death. Deep down I don't think you really want this to happen. That's the depression talking. But as you can't see a way forward for yourself, you are blinkered in your thoughts & reasoning, which isn't allowing you to see your options clearly. You need to take a breathe, step back before you do anything else.
Is there a colleague (perhaps your ward sister,) a relative or friend you can share your worries and feelings with. The hospital will have a PALS service & a chaplain, both who you can talk to in confidence. You don't need to be religious to talk with the chaplain. You can contact PALS or the Chaplaincy department yourself via the hospital switchboard number. In my experience, both are likely to get back to you the same day if your phone-call is not immediately picked up. Why don't you explore those options.
Alternatively, you can ask to speak to one of the ward medical team, or a DSN from another department. Confidentiality will be respected. Doing nothing cannot be an option for you now hun.
The first step to accepting you need help seems like a giant one. But, you know, and I know from your words on here, you are clinically depressed, a mental illness that will not go away on its own. Ideally you need to be assessed by the psychiatric registrar or consultant whilst you are in hospital. The hospital will have a mental health specialist liaison nurse...an excellent resource who can quickly access services for you. And, who you can speak to upon request. Even just to see what help is available to you. Mental health outpatient departments offer support and/or day services if necessary, and can be accessed upon discharge from hospital. But, you need to explore these options now.
Stop suffering Emily. Make the first step to getting yourself well. The first step is actually saying to someone "I need help!" X