Oh god! $64,000 question. Thanks, Leeann. No, really. Thanks a lot.
We all go thru this, especially in tournaments. Bottom line is: I think you just have to go through it a number of times, until you learn to get over the nerves.
But, that said, i like Annie's suggestion. A nice clean serve that can give you some confidence can go a long way.
I find that when I start getting tight, it helps me to stop and take some deep breaths -- a set number. Usually five for me. Slow, don't rush them. Inhale deeply, exhale. It gets rid of the physical tension and it breaks your destructive train of thought.
People tend to rush when they get nervous or anxious.
Then, I tell myself to just focus on watching the ball and watching it all the way to my racquet. I trust that my strokes are alright and that I will stroke the ball properly if I am focused on watching the ball -- really seeing. And, it distracts me from thinking the destructive thoughts.
And, at the risk of getting too 'zen' here, might I suggest some mental visualization?
There is a famous story of a POW in Vietnam. This guy was imprisoned for like six or seven years. He loved to golf. But, of course, he could not. So, every day, he would play 18 holes in his mind. He would really focus on the game, on his form particularly. He would hit the ball just right. He would not let those fears of shanking the shot intrude, and if they did, he would practice that shot over and over later.
Anyway, it was his solace and his escape of his plight.
Interestingly, when he got out, his game had improved, greatly, without even practicing the physical part of the game.
I think you can do the same thing with tennis. Practice imagining a tense point, or a tense game, and picture yourself hitting the right shots and hitting them well. It retrains your subconscious (which is what undermines you) and helps to banish doubts.
Personally, I think making it to the finals is pretty good. Congrats. Again.

P.S. I just found a link to the POW story. It is on a mental toughness group website.
http://www.mentalgamecoaching.com/IMGCAArticles/MentalToughness/MentalToughness.html

Completely agree. For doubles, I only do my flat hard serve once a set when the opponents encroach too much on the service box. 99% of the time I stick with a very spin serve or as you do an over the head topspin kick serve. I change these up to keepem guessing. Works well for me also.
-Mark

Unfortunately, I have had brushes with death and I didn't improve my game like that guy! Lol I was frustrated with my record this USTA season, but it is my first season playing singles in many years. But last week I played a 3 hour match and won a third set tie break 12-10. At one spot I started to mutter to myself, do you just not want to win? I realized I was getting scared of going for my shots when it was game point, in the first set I lost my serve after 12 deuces -11 times I had ad in, after losing the first set and being down 2-4. I decided if I was going to lose I would lose big, like Billie Jean King says. I won that set 6-4. The great thing about the tie break is that thanks to leaving 90 pounds behind, it was my 20+years younger opponent who finally showed signs she was just too tired. I have joked that I really need a tennis shrink to answer the choking and other mental issues. But to be totally honest I would rather just hit and get a great workout than play a match.