1. When you're not at practice picture yourself doing it perfectly and without fear on the tumble track with no one spotting you. Picture this at practice too, but definitely do it when you're not at practice. You'll be surprised because you'll probably picture yourself with fear and with a spot at first, it takes practice to picture yourself doing it the way you should.
2. At practice, warm up really good and thorough.
3. Limit yourself. Tell yourself that you won't get spotted more than once (or however many you need to be spotted on). Then promise yourself that you won't leave until you throw it by yourself on the tumble track just once. Then the next practice, promise yourself that you'll do it twice before you leave. And so on. . .
4. Try to find a way to relax yourself. If you get too frustrated, sometimes there is no point in trying to keep throwing it because it leads to more frustration, sometimes tears, and then it can lead to injury. When you get to this point, work on a fun and new skill. Once you're relaxed again, work on your bhs again.
These steps have worked for me. I am a tumbling coach that is still mental from time to time. To me, step #1 is the best one for me and it's the easiest! I get amazed every time I try to picture myself and I see myself doing it all wrong! It's like I'm expecting myself to fail. Believing in yourself is the best tool you can use to get over this. The second best tool: persistence.