Always tricky giving advice without seeing you, first hand.
From your post, it sounds as thought the run up may be the cause of your issues - once you start thinking too much about overstepping, where your feet should be etc, then rhythm disappears. I suspect that if you can smooth your run up, then your rhythm and control will return. Has anything else changed (eg are you loosening up more, or less, than before? Bowling from a different end, such as up-hill?).
There is a caveat that tinkering with your run mid-season is not ideal, as it can take some work to get right. That out of the way, it might be worth trying afresh with your run up. One popular method for this is to go onto the outfield (nowhere near stumps, just an open space) and mark the start of your run. Ideally have a friend with you. Then go through a comfortable run up - building pace steadily (I am assuming you are not bowling spin). Once you are at a comfortable pace, bowl. Mark where you have bowled from (ie front foot landed - this is where the friend helps). Measure back to the start of your mark. Try this new run up to get it 'grooved'.
Alternatively, mark your usual run up, in the nets (perhaps without a batsman) and try to hone it. Again, with a friend. Try to find a spot to put a second mark - partway through your run, that you 'hit' when your run is working smoothly. After several more attempts in the nets, this second mark should then be marked in a match - once you hit the mark, you know your run up is 'on cue' and will hopefully reduce your anxiety about over-stepping. Once the anxiety goes, then you can focus on where the ball is going and what you are trying to do with it.
Good luck.
David