Firstly, I very much agree with those who have suggested going back to your GP. Anti-depressants are not a cure-all, and certainly do not work for everyone. Most people respond better to medication when it is accompanied by some kind of therapy. Most areas of the UK have IAPT (Increasing Access to Psychological Therapies) teams, which do exactly what it says on the tin! They may support you by working with you to increase you motivation to take control of you diabetes and setting small, realistic steps to work on. But these teams require referral from a GP or other specialist service (like DSN).
One way that I think I came to grips with my diabetes was to think of it as a lifestyle choice rather than a disability or illness. Don't get me wrong, when I was 13 I rebelled, I used to skip school dinner so I could buy cannabis and alcohol, I'd forget that I was diabetic until my sugars were so high that I felt really ill. But, I was lucky to never be hospitalised and came out of my phase without too many problems. I still have highs (recently 30+) and lows, forget my injections, and get fall down drunk, but these are very rare, and I tell myself I'm still young and too easily influenced by my friends. Now, my life is more about good health. As a young adult I want to be careful with my diet and take on exercise challenges (recently I ran the Derby Kilomathon (26.2km)). Getting diabetes actually helped me to become the person I am now; I see some of my friends that didn't grow out of the 'cannabis phase' (their health did not seem to pose such an immediate danger to them) and now I'm their drugs counsellor...
What I'm trying to say (but going the long way about it) is that focusing on how diabetes can be a good influence on your life rather than such a negative will help you to accept the changes that are inevitable in your life. Counselling can help with this. In the mean time, make small steps and small changes, don't put too much pressure on yourself to get it right straight away. Test once a day if you can, or inject once a day. And when it comes to your Mum, talk to her about practical things that she might be able to do to help you test/inject more regularly (without seeming like she's on your back or nagging). Be positive, you CAN succeed and regain control of you life, health and mental health.