It is difficult to change the result by just experiencing it more often or by one magic thought or action. To make lasting changes in your game to help perform during stressful situations, you need to develop the right mindset for every shot. Not just the stressful ones.
Develop an effective per-shot routine. Not just what you do when you get to the lane. But also what you do when you finish a frame. For instance,
1. After you finish a frame, take a second to evaluate what happened compared to expectations. Not in score, but in approach, release and ball reaction. You can throw the ball great, hit your mark and get a bad result. Don't let those moments ruin you. Use the information from that frame to help you decide what to do on that lane next time.
2. Relax and unwind. Take a moment to distract yourself from bowling. Talk with teammates about new equipment, other sports, the attractiveness of (or lack of) the bartender. If you think bowling 100% of the time, mental strain will creep in and lack of focus will result.
3. Before your next turn, take a moment to re-enforce your decision on the next shot. This is your chance to convince yourself the shot you have chosen for the next shot is the best shot. Don't let negativity get in your head. Based on your personal experiences and what is happening that night, your next shot IS THE BEST SHOT FOR YOU.
4. Develop pre-shot routine. This is important so you can have a consistent physical approach to the game. Whatever it is, do it every time. This is the most personal thing and can even include idiotic stuff, but it is yours. Keep it the same for spare shots also.
5. Execute the physical aspect of your chosen shot to the best of your abilities. Once the ball has left your hand, you have no control over it. If you strike, great, react to the great shot. If you have a bad result, that's fine, react to that and move on. Learn from the shot and start over at number 1.
I know this was long winded, but once you remove the results from the game and focus on the process, you will become more consistent and more even-keeled. Concentrate on the parts of the game you have control over and don't worry about the parts you don't.
Bold for emphasis, and I could not stress the underlined enough.
Keep in mind this. While everyone else may be so excited and amped up to see the 12th shot, you are in no obligation to hurry yourself up to please them. Take as much time as you need
, especially when you need to invoke calm. You aren't bound by a shot clock, like the PBA is.
My first 300 came when I didn't realize I had the first 9 in a row. But what I did know was that a tornado was coming.. literally. Sunny day outside, people were outside playing sand volleyball, but inside, they had the TV on a football game, but kept getting preempted by the weatherman talking about a Tornado Watch being issued. I took my mind off bowling by watching that and seeing where it was. That went on for a minute before I threw my 10th. Then I went back to looking at the the weather.
Wash/rinse/repeat for the 11th, then finally, the 12th. Each time I got back to the approach, it was as if I didn't even know what the situation was. Went through the my pre-shot routine (as mentioned above), and threw the 12th as pure as I did the first.
In Omaha during the 90s, the PBA tour wasn't as prominent (only a regional tournament during the summer), but the Women's tour was always there. In my time watching them, I noticed so many CD and tape players with headphones, books, etc.. anything to take them out of the stress of the next shot so they could perform as if it were the first shot in league or one with the least amount of pressure.
If you could get into that mindset, and you'll be the most relaxed you could be, and perform the best that you could. And finally, commit yourself to that shot. Throw it like you mean it. You didn't have any problems with the 3rd or 4th shot; if you're relax and convicted in the move and shot you're about to make, it should feel no different than that 3rd or 4th shot. Throw it like one of those, and you've done your best. That is all you could ever ask for, let alone what anyone else could ask of you.