There are two issues to deal with.

First, Treating the anxiety that causes startle to have such a hair trigger. This may benefit from some profession guidance to desensitize this response. Training such that you are walking down the street and know somebody is going to attempt to startle to and you are using relaxation techniques to be ready for it. Some get startled by touch, especially to the head. Having people in your house walk by and lightly touch you. You first might want to practice sitting with your eyes closed and waiting for people to quietly do it.

There are counselors/therapist who know how to do such exposure therapies.

Second, When you are startled, you need to have your rescue technique to settle your body and mind. The same therapist may be able to help you develop them.

The goal is to prevent a simple surprise from turning into a bodily response.

Having a rescue med available can help, especially in the early stages of developing strategies. Propranolol (a beta blocker that settles blood pressure and similar issues) or clonazepam ( a benzo anti-anxiety med) can work. They only need to take the edge off. Occasional use of either should not be a problem for most. Often, a very small dose is all that is needed.

It can help to address the startle trigger directly and discount it as just a surprise. "Ohhh. You surprised me. Don't do that." It acknowledges the surprise and puts it in context and redirects the issue.

It may take some time but it can be done.