Please... all you seasoned aviators... tell me it gets easier!
Well, forgive me for stating the obvious, but... of course it gets easier! Every circuit you do, a few more little things get wedged into "muscle memory" and don't require conscious effort next time. After another few hours, you might even have enough spare brain power at the end of the circuit to land...
(well, that's how it was for me!)
Don't worry about it. Sounds to me like you're slightly misjudging the trim at the start of downwind - or if the aircraft is gaining height at the same time as maintaining cruise speed, you've got slightly too much power. Make a mental note of this, and next time you get to downwind, you'll remember to correct this, and the height will be bang-on all the way to base.
Of course, if you're anything like me, the concentration of getting altitude control right will cause you to screw up something different, something perfectly simple that's never been a problem in the past, like forgetting to turn on carb heat or forgetting to call downwind. And you get frustrated, and the next time you get that right, but you screw up the height control again... and the next time you get them *both* right, but nearly drop the aeroplane onto the runway from 10 feet... and so it goes on. And they seem to think I'm a good student!
And then suddenly, you'll have done a circuit, and nothing went wrong. The next two will be disasters, of course, up to 1250ft on downwind, but then you'll do another good one. And then a couple more good ones.
And then, just after you call "downwind touch and go" on your second circuit of a session, everything going beautifully, the instructor will chip in on the radio with "correction, downwind to land"...
