Letter from a birth mother...

So, a little background information: I am 21 years old, a college student, and I recently found out that I am months along into (an unplanned) pregnancy. My boyfriend is the father, and we have settled on adoption. I am still conducting research into open vs. closed and just trying to figure out what would be best for our child. I personally am leaning towards openness but really just trying to evaluate the degree of openness that would be best for everyone.

In our talks about adoption, we have considered writing our child a letter (written individually I believe). My letter would include why I chose adoption, why I chose whatever family we end up choosing, and just telling them a bit about myself, and maybe even including the fact that this wasn't due to carelessness (we were consistently using 2 forms of birth control- pill/condoms) but that we (the father and I) agreed that the best option our child had was to be placed into an established, loving, and ready couple.

Do any adoptees or adoptive parents have experience with a letter from a birth parent? Do adoptees wish their birth parent had/hadn't written them a letter? What would your ideal letter include/exclude?

Much thanks in advance for your feedback!