I’m sure this is very painful for you. But I’m not sure things would be different if you had told him the truth about your past involvements in the early stages of your relationship. His reaction may be about the fact that you broke his trust, which is serious. But I’m wondering if he is also struggling with the fact that you were with other men before him. If that is the case, you need to be talking about the difference in your histories and your values as well.
I’m sorry your boyfriend won’t see a therapist. You can’t fix his anxiety. If he could do it on his own, he would have done so long ago. From what you shared, his level of anxiety is serious.
Unfortunately, there is little you can do at this point to settle him. Trust is easily broken and very difficult to reclaim. It is up to him to find a way to understand your motivation and to then forgive you if you are to move forward in this relationship. If he is unable to tolerate the fact that you had other relationships, that too is something for him to work through.
You can’t undo your history. He may be able to come to understand that history is just that — history. What matters is what you learned from it and the fact that you are choosing him now.
I wish you well.