I had some troubles with this skill as well. My coach always told me not to straddle too early in my kip cast HS and to shift my hands over the bar when I got to front support.
As for mental tips, there are a few visuals or little clue words that I could say to myself before I did the skill. The clue words were "shift (my hands)", "lean (over)", and "cast up (I wanted to think about getting my hips up super high before I started the straddle)". I also thrived on visualizing skills, so if you are a visual learner, I would recommend this technique. Before you do a skill, picture in your mind's eye how it would look and feel to do it correctly (I.e. see yourself doing a super high cast and leaning over; picture what part of the floor you will look at when you lean over through front support, etc.). Also, if you have a low strap bar, practice lots of kip cast HS on that.
(This video is also a good resource that breaks down the movements of the skill into baby steps)
Cast HS
Hope this helps! As always, all this advice might've worked for me, but there is no guarantee it will work for you. Good luck!