Put a second magnet on your wheel. You'll have to pay a small weight penalty but after you factor it in you'll almost double your speed.
Or
You're having some days where you're pretty quick, but they seem to set you back because the next day you're always slower than your baseline. What that sounds like, is you need rest days. You go out and exercise, you stress your body, your body decides to get stronger in response, but the actual getting stronger happens while you rest.
Go a little easier. Since you're new to cycling, you probably need to build fitness. Go for long but moderately intense rides, don't do any (or at least much) sprinting, just put a lot of miles down and keep pedaling the whole time. Do a lot of that, with occasional hard days (lots of hills, lots of sprints) and occasional rest days. Give it some time. Notice your body get stronger.