I'll try a response now - obviously this is very personal to me, and it's very different for other parents
Letterbox is definitely supposed to be for the childs benefit. It may benefit the birth parents as well, but that's not supposed to be the primary goal of it. On the other hand, some birth parents find the letters very difficult, and so they never write back, and it's all one sided. I think it's easy to assume that all birth parents benefit and it's for them, but in reality I think there are a relatively significant group of birth parents who it doesn't benefit, but it's set up anyway in the hope the child will be helped by it. On the other hand, I know there will be birth parents out there who count down the months and days till their next letter
I have written a lot of contact letters to siblings as well, but I find those to be very different to birth parent contact letters, very different emotions attached
Letterbox brings up a lot of emotions for me, a lot of feelings. Writing it is difficult, recieving it is difficult. However, I know that letterbox has benefitted my DD2, so it is has been worth it (I no longer write to birth mum) while it lasted, which was many years. If you can see letterbox benefitting your child, then that makes it easier because you can see the purpose playing out in front of you, and you can say 'this is really going to help my child' when you struggle. If your child is too young to really comprehend it, or doesn't seem to be getting real benefits from it, it can be harder
It does force you to confront not being the only mother in a head on way. Of course that's the reality of adoption, your child has a past, and birth parents, but when you write a letter, you really have to confront that head on, and that also happens when you recieve a letter. Now for me, I'm okay with this, but I know that some adoptive parents find that aspect of letterbox very difficult
For me, the hardest thing is confronting my feelings about birth mum. I'm really talking about letterbox with DD2 and DS birth mum here, not DD1's birth parents (she hated contact, I hated it, so I stopped it). I have many mixed feelings about DD2 and DS birth mum on the other hand...worry, sadness, anger, sympathy, irritations...it can all come to the fore at once
I am sure if DD2 were a relinquished baby, writing would be different, but of course she's not. She was removed for serious neglect and abuse. The letters I would recieve from birth mum sometimes very forcefully reminded me of this - the dysfunction in her life was very evident in her letters. She also clearly had absolutely no understanding of how to raise a child. On the one hand, she would write that she was happy to hear that DD was happy, and she thought I was a great mum, which made me teary and feel very grateful to her (for being strong enough to write that to me when her children were taken from her against her will and she wanted them with her, not with me). But then DD and I would ask to hear some of birth mum's memories of DD, and she would write back that she was happy to share...and I will never forget this letter, one of her memories was of how she and some of the kids (DD included) liked to spend lots of time having fun and they all liked to have a drink together. Apparently DD2 loved WKD and she was happy and had funny when she drank 'a bottle'. It didn't help that DD2 herself thought (and probably still thinks) this was a really cute story. I don't think getting a 3/4 year old child tipsy/drunk on alcopops is cute. Obviously. But that's one good example of having to confront DD's past in writing.
And there's the affect on DD. She might recieve a letter, we would go through it together, and then she might cry and say how much she missed her mum. And that's hard, because I have to deal with the fallout. And then my own feelings related to that - eg. why does DD love and forgive her every single piece of neglect and abuse, no matter how awful the details? I hate dealing with all the problems caused by birth mum, while she never has to see it. I'm so sad that birth mum had such a bad life that she thinks like this and lives in such dysfunction. And so on and so forth. If DD were going through a bad patch when i wrote, it would be harder, because I'd be thinking 'what I want to write is, "she's really struggling now, and that's actually your fault because you x,y,z, did this, exposed her to that, and I'mreally angry right now". But obviously that's not appropriate, and I want a good letterbox relationship, so I'd try extra hard to get the right balance in the letter
Writing is hard, because you are always trying to get a balance...you want to write something good, and you strive to get the right balance..eg. do i write about difficult issues like diagnosis or therapy or not? You are very conscious about what you are writing (or I am anyway!)
I'm sure I've made it all sound really difficult. I can't speak for anyone else, other adoptive parents will find it a very different experience of course, and I expect you would get a completely different response to someone else
But as I said, I know letterbox has been of benefit to DD2, so for all the difficulties, I certainly don't regret writing to her, and I would do letterbox again, because ultimately I am doing it in the hope that it will be of benefit to my child, and I am willing to go through any amount of issues if it will help my child