I used another typing test which was in Dutch and for some reason I hit ±60 wpm with English tests and ±70-75 wpm for Dutch typing tests. Very strange. Have any other people experienced a similar thing? I wonder why this is.
Perhaps because most of my typing is done in Dutch and my hands are more used to it?

Woah. TypeRacer tells me I'm nearing 80 wpm with my typing tests now. And these are English typing tests, so my English typing speed is truly improving a lot.
I keep hitting 80 wpm every now and then but then I can't keep it up for long and usually average around 77 wpm. I'm almost back on my old Qwerty level! (Which was 85 wpm)

And it has only taken me four months to get there. Over the last few days I've really been improving fast, so I'm confident that I'll reach it soon, and possibly even surpass it.

So. Much. Better. I feel much more comfortable typing like this. I have noticed that I don't make half the typing mistakes anymore that I used to. Also, I've become a touch typer who is able to type with ten fingers, instead of three. That already helps a lot, and then there's the comfortable feeling from Colemak which I really like. It feels so effortless. So enjoyable.

Sometimes I still get these periods in which I keep making mistakes and more mistakes all in a row, as if when I make one mistake I have to make twenty... but most of the time I'm fine and then I finally feel like I'm getting really fluent: I can almost keep up with the speed of my thoughts now. (or perhaps I simply think too fast? XD)

A combination of the two. But I wasn't able to learn touch typing properly on Qwerty because I was used to doing it with three fingers. I'd automatically switch back as soon as I stopped thinking about what I was doing. This change to Colemak allowed me to relearn to type, as it were.

Thank you :-) I'm happy it worked out so well, too.

Edit: Oh, this makes me happy. 83 wpm with 98,9% accuracy. Twice. And I've been getting ~75-80 wpm scores easily all day.