I understand that feeling, I even get it when shopping if I see someone that may/may not shun me. It has upset me that I seem not able to control my feelings.
This last week I had a bit of a revelation, I practiced finding the centre point with mind body control, its a principle of most eastern practices, tai chi, yoga etc but it really worked for me. The session involved thinking of someone or something upsetting and allowing your partner to push you gently on your chest, it unbalances you. Then you are asked to concentrate on your area below the navel and say to yourself, remain centered. The partner then pushes again on your chest and it was amazing how the body actually stayed firm. This practise can really help in overcoming negative feelings.
When I told my 13 year old, he just laughed at me and said, oh that old thing, I use it all the time in my hapkido lessons ( we enrolled him as soon as we left the witnesses some years ago). I was so proud of him as he is the most balanced, calm and happy kid I know (even for a teenager!).
So I suggest you search for a video that teaches this technique and try it out...it does not have to be in a witness/fear situation it can work in any area.
One teacher of the technique is Angela Hicks, she does very good presentations.
Hope you get over your bad feelings
Regards
Chicken little