When I get close to the crease, I start doing zigzags with my run-up.
I would put this down to poor running technique. Fast bowling is about channeling energy in a straight line towards the batsman. There's no reason for you to be doing zig zags. I would focus on improving your running technique and running with proper mechanics.
I would then take those same mechanics into your run-up and bowling. Be patient with this though, if you've been doing zig zags for a while, chances are that it's part of your muscle memory so make the transition slowly and be patient as it will feel awkward to begin with!
All the best.
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