Seems to me that you may me too "worried" about the outcome of your serve. That usually makes one choke.
The solution is to observe your mental state when you practice and become aware of your thinking - if there is any at all. Do you have any expectations when you practice? Probably not. You just hit serves and observe what's going on.
The next step is to become aware of your thinking - or your expectations or your reactions to misses - in matches.
When you become aware, you have a choice. When you are not aware, you run on autopilot.
What do I mean by choice?
You can choose whether you will be thinking about the outcome, mistakes or expectations or whether you will focus only on the process of serving. You need to let go of the outcome and just serve - the way you serve in practice.
How do you convince yourself that you won't think about the outcome?
In my next post.