As I understand it, amends are made for the growth of the person who is making them--your XABF, in this case. If he has done the work he should have done, it will make no difference whether or not you actually ever receive or read his letter. HIS recovery is in no way dependent upon YOUR reaction to his intent to make amends.
You would certainly not be the first or only person to refuse to hear amends from someone who has hurt them in the past, and you certainly have every right to refuse any contact w/him. Just as writing the letter is for HIS personal growth, the choice to receive it or not is totally YOURS. You have no obligation to do so whatsoever.
Also, in regards to his apologies being something that might aid in your healing, be prepared for the fact that what he feels he needs to make amends for, what amends he chooses to make, and how he wishes to make those amends, may be far, far different from what you feel you deserve or need to hear. If it would cause a setback or resentment in you to receive the letter and then not find it adequate, you might be smarter to simply refuse it.
This isn't really answering your question about why you feel anxious, but it seems to me that if you are feeling enough anxiety to post here about it, you maybe should consider whether you're still feeling that pressure of somehow being responsible for someone else's happiness/recovery/life that so many of us with A partners do feel. If that is indeed the case, you might want to reconsider your decision to receive the letter. He will recover (or not) regardless of your decision. The person whose recovery you need to be most concerned with is your own--take care of yourself.