feeling frustrated

We adopted through fostecare and wanted most of our contact with dd's birthfamily to be through letters, birthfamily wanted most of their contact to be through visits.Yet we have no contract for continued contact,it is completely up to us. We've tried to come together and compromise but I feel like I've bent over backwards over the years to give them visits like they "need", but anytime I beg and plead for letters for my daughter, they just don't come through. Our dd has been having a difficult time with visits, especially over the last couple years and because of it (and a bunch of other reasons) we've had to decrease visits to 1-2 times a year. Decreasing them has helped her by giving her more space, but visits are even harder for her because all those emotions come back to her and she really doesn't know them, and she really want to know them better. About 2 1/2 months ago I spoke with birthmom about it and again, and requested that she start writing dd so they can build a relationship between visits, hopefully making visits more comfortable for dd. It caused an arguement because Birthmom always fights with me on "opening up" and "sharing", but especially when it comes to "writing". After our arguement I wrote her and birth grandma a very kind letter that talked about my dd and her need to build a relationship with them through writing. I explained the difficulties we are having with visits and our hope that letter between visits will help make the visits easier on dd and also allow her to get to know them better. Well, birthgrandma wrote me right away saying she would and shortly after we received a letter in the mail and our dd really loved it, but we still have not received a letter from birthmom. I was becoming really angry and resentful, knowing that we have always tried to give them what they need and want in the relationship but birthmom has always fought us on the issue of communicating and especially writing letters. The very thing that we wanted to be the foundation of contact in our relationship. I ended up messaging birthmom again and asking her "why" this is always an sensitive issue for her. She just stated that it's hard for her to know what to write and what to say,so she just doesn't do it. Part of me wants to just lay down the law and say "hey, if your not willing to give dd what she needs in order to have contact than maybe we need to stop having contact". But I don't want to approach it like that. I want her to do this for dd to make this easier on her, not because she feels threatened. However, if things don't improve and dd continues to have a really hard time with visits it will be our only choice since she has given us no other option to try and make things better. (oh, no she did recommend an option. She asked that she have alone time with dd to take her out and talk or to have a sleepover at her house.) We have never had unsupervised visits and certianly wouldn't have a sleepover. Aside from the fact that the last several visits we've had dd has broken down in the visit and come to me for comfort, and the last time we saw them she wouldn't even let go of my hand she gave me the death grip letting me know she didn't want me to leave her side. Having a private visit without me there wouldn't even be an option. Yet, it's another example of birthmom pushing for more contact but refusing to give us what we've asked for as far as contact goes. How do you deal with a situation like this?