If you are afraid of what he might do to you contact domestic abuse hotline and they will help you with a plan. I don't have personal experience with being afraid for my physical safety so can't advise but urge you to take that seriously.
If you are uneasy creating this boundary - that I have experience with. That feeling of unease and uncomfortableness with creating and establishing personal boundaries. It wasn't fear exactly but my body reacted with similar physical symptoms when I thought about creating those boundaries. Sometimes I still do.
This is one thing that helped me. Always remembering or reminding myself that feelings are not facts. I might feel uncomfortable but my logical brain tells me it is OK (right and good even) to create boundaries and take action to uphold them. Just because it feels 'bad' does not mean I should not do it. Sometimes my physical reactions (inner panic, turmoil, feelings of fear or doom) are based on very deeply held dysfunctional patterns not related to the issue at hand. I'm not sure I'm making sense. --- You have the right to end a relationship. You have the responsibility to take steps to protect your own best interests. It is OK to change your locks, codes, block numbers and emails, etc. Those things may cause a feeling of uncomfortableness or anxiety but use your logic. Your brain can tell you that these things are good and healthy for you to do. You are not hurting anyone, taking advantage of anyone, ignoring your own needs, or doing anything at all wrong. You are simply looking out for your best interests and protecting your boundaries. That is a good thing - a thing only you can really do.