Hi man,
0,5 is a huge dose, wether over 10 hours or less. It's just not healthy. A clean roll should be a single dose below 250mg, if ur taking that as much as 0,5 you can ask yourself some questions already.
Coke and MD are not a good mix, it probs was ur coke that was cut with some speedy shit and made you feel like shit.
I must say UK speed is crap, at least the stuff I've tried... Here I get moist paste that reeks of almonds and that smell of car paint, 100mg will last you 12 hours - that is speed.
When you take too much MD, the entactogenic effects dissapear and leave place to a speedy effect because you serotonin, noradrenaline, dopamine is depleted. Thats why rolling more than 6 hours will make the comedown harsher and harsher...
Take a good break, focus on your health man.
Peace