Collyi,
What support you would find would vary incredibly with what area you live in sadly. Bit of a postcode lottery
With the letters, there should be a 'letterbox agreement' or similar that was written when your son was adopted. This should say in it when letters are sent, and how they are going to be signed off, and who writes to who. You should be able to view it if you wish. I would if you are able to, phone and ask to see it or it be read to you.
I'm not sure whether you have written any letters already, but usually letters aare written between the first parents and adoptive parents, not between the child and first mother. So you would be writing to his parents. Depedning on how old your son is he may or may not see the letters. You could include (just some ideas)
-Any major family news (deaths, births, marriages of people who are close relatives)
- Any positive news of things you've been doing e.g. "I have a new job and I love it" etc
- Comment on the letters you might have recieved, eg. "I can't believe he is that tall now, he maybe takes after his grandfathers, both were over 6 foot as adults"
- Ask questions if you really want to know the answer to one particular thing eg. "How is x doing in school now?"
I know it must be really hard to try and work out what to say, especially as you haven't had any guidance at all (you should have some). Don't overthink it is my advice, and try and be yourself as much as possible, because your son may read these letters one day. The letters I get from DD2 and DS mum, her personality does show in them. They are very varied
There's not a lot you shouldn't write. I would just remeber you're writing to the parents usually, so gear your comments towards them.
With support groups, some areas actually do have support groups for first mothers to come along to. Some don't. You could phone again and ask if your area is one that has a support group or not. I would also recommend trying to get counselling if you are struggling. There are severa organisations and charities who could maybe offer that, or you could see your GP
And there's nothing wrong with sticking around here. We're a friendly bunch :) There are other internet forums which have special boards just for first mothers (the one I am part of is incredibly welcoming to everyone)
I hope that helped even a little