Hello, I'd like to help you with your question.
What you are feeling is very natural in response to what you are going through. Your fear is not what would be consider a phobia or unnatural fear. You are responding in an emotionally healthy way to the stress of your situation.
What you don't want to happen is that your fear does turn into an unreasonable fear, one that hurts your ability to live a normal life. In order to help yourself reduce your fear, try the following:
Start by talking with your doctor about options you have to address your allergy medically. One way that I can suggest (with your doctor's approval) is to ask for an epi pen. Many people who are highly allergic to bees or food carry epi pens just in case they are stung or accidentally ingest the wrong food. This can bring you peace of mind just in case something does happen.
Also, take as many precautions as you can to stay away from shellfish. Talk to your husband about only eating shellfish when you are out of the house or let him wash the dishes instead. Also, make sure any restaurant you go to knows that you are allergic. If it helps, talk to the manager and explain your needs. This will help you feel more secure that others are looking out for you.
Work on reducing your stress level so you can feel calmer when dealing with this problem. Here is a link to help you find ways to improve your skills in reducing your stress:
I hope this has helped you,