You shouldn't slow down from improved form. I would challenge your assertion that your form was "terrible" and is now "decent."
Perhaps you tried to change your form and actually made yourself less efficient. Scratch that, you clearly did, or else you stopped training and only changed your form, just making yourself in worse shape.
Improvements in form should take seasons, not weeks, to improve. If you "improve" your form faster than this, you probably haven't actually improved yet, you are just forcing yourself to run differently, in what may or may not be better form, but if you are forcing it you are probably tight and inefficient.
Sometimes as a coach, I get athletes who try to "learn" on their own with youtube and such. The result is usually that they learned somewhat of a caricature of the correct technique and actually get slower, injured, or both.
Consult your coach on your form. If your coach is bad, this won't help, in which case, good luck.
You also don't have many details in your post. More information could be helpful. Maybe you're going through a growth spurt, which can temporarily slow you down, maybe you are fasting for ramadan, which can temporarily slow you down (or temporarily speed you up if you are playing with anaerexia), maybe you are coming off an injury, which is what prompted the change in form, maybe you haven't been running much and your teammates have, maybe you're actually a robot and one of your wheels fell off. You leave us guessing the answers. My answer is the best I could come up with given the information you just barely let peak through onto the internet.