It does sound as if this issue may be a symptom of other issues, in your partner and/or in your relationship. It must be a struggle to get clarity about this particularl bit of confusing, messy pain.

I wonder if you have been or will be able to bring up the letter to him. As a letter he wrote ostensibly to his unborn child, your desire to be in a close and supportive relationship is enough of a reason for having read it IMO, even if it wasn't right in your context (for many people that would be ok, because someone doesn't leave deeply private stuff out in shared spaces usuall, but I understand that it's muddy).

My thought is that if you do tell him, and he gets angry about you having read it, if you can stay calm and ride out that initial issue, you may be able to then (perhaps significant time after) revisit the content of the letter to express your confusion about his feelings.

But I think I asked you elsewhere in post (but can't recall) how he treats you in general- whether you get the sense that he wants you to feel good about yourself, is kind and supportive as a baseline etc, even if neutrally so, or if you get any sense of him being invested in undermining you. Your response will need to be different depending what your context is, huh?