You sound like me a few years ago, its a frame of mind that plenty of us can identify with. Its one of the reasons I think there needs to be more support and education for young adults trying to cope with diabetes- there simply is not enough accessible support or information out there. When I have a more relaxed headspace (ie, better control myself, hopefully less stress about it, and a calmer outlook) I am going to get involved in my local Diabetes Uk branch and see if I can help organize some extra supports, in particular for those from about 15 to 25.
It is bloody frightening and also pretty **** tiresome to constantly keep trying to manage diabetes at times. When I came out of my irresponsible phase, and began the process of reclaiming control, I had to use an "action before motivation" technique, or I wouldn't have done anything. I felt overwhelmed and ****** off, but decided I'd start trying anyway, despite not feeling like it, so that at least I know I'm trying...actually I still do this lol. I have a lot of anxiety wrapped round my diabetes, so sometimes I almost freeze, and its so tempting to ignore it. But no good comes from that. So I keep trying- I haven't got it right yet, but I'm hoping I will soon!
So, about motivating yourself- I would say if you take a small step straightaway, such as recording your blood sugars (if you don't already), and to re-familiarize yourself, perhaps start reading good, clear, informative material on how the body, insulin and food interact with each other (I know you probably know all the stuff in theory, but its surprising how much you can learn if you find a scientific breakdown which is written clearly!) Another step would be to book in with your clinic or diabetes nurse soon, for a chat about where you would like to make changes. As you become more actively involved in managing your diabetes it becomes easier to motivate yourself to do the next thing, and the next thing
I hope this all goes well for you!