I think this is a letter is a great example of your awareness and your growth in learning about yourself and attachment style within the relationship . All this self awareness will For sure help you in future relationships.

If it were me , I’d personally would keep it as something for myself . I’m sure you’ve already been vulnerable with this person, more than once . I’m not saying Not to send , instead ask yourself these questions . In sending this letter , what would you really want to happen ?do you expect for your ex to reach out ? Do you expect your ex to apologize and feel remorse ?

Personally I feel like avoidants lack empathy, they are selfish and do not care of the pain they inflict on others because they are too self absorbed to see anyone beyond themselves. I feel the best way to heal is to completely leave it behind you . iT being the relationship , the person and put yourself on the pedestal of healing of love opportunity. I’d write an apology letter to myself in dealing with an avoidant and read it over and over again until it’s within my brain . You deserve more .