Hi RT,
You are doing amazingly well so far, but you do need to stop being so hard on yourself.
One hour and 20 minutes is not a bad time for a 10k, and to be honest I have been at races where there would have been many people finishing after you, so don't be disheartened at being last - someone has to be, and you still did it quicker than all those people sitting on their sofas that day.
If you struggle with motivation or become frustrated easily, some kind of visual aid works wonders. I know it sounds odd, but if you draw up a wall chart or training calendar, you will get a great kick out of crossing out each day as you complete that days' challenge - whether it be a run (but dn't run every day,) or a diet-related challenge such as "make a healthy soup for lunch."
At the end of each day, or month, seeing a wall chart full of acheivements is very rewarding, and if you can keep it up for long enough, fitting in the new changes soon becomes second nature.
Having some kind of reward system in place also really helps - something for the first week, month, and so on. Not a food treat mind, but a new top or haircut, whatever makes you feel good.
Hope this is of some help to you, best of luck!