You would want good control as a pitcher so, "to become not-suffering in control" or "attain good control" is the default way of reading this.
but what exactly is コントロールに? i have to resort to reading it as コントロールは/が...
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This に marks the cause/reason of one's feelings or pychological reactions. For example:
So コントロールに苦しむ means "to suffer from (one's bad) control". コントロールが/は苦しむ means something like "(Mr.) Control suffers", which makes no sense here.