Hi Strawberry, yes I've had couples counselling with my DH (he was then my DP) and it was amazing. Tough, gruelling etc but he had problems listening to me because of stuff that happened in his childhood and reacted badly when I tried to bring things up - like I was getting at him or criticising him. Once we started going to counselling, it gave us a forum where a third party could ask questions rather than it just being me.
FWIW I convinced him to go to counselling by stressing it wasn't about anyone being at fault but about us, as a couple, being happier and finding ways to get on better.
I am not sure we could have worked through the issues that were scuppering us without an objective counsellor. But if you want some tips, I can try although they might sound quite cliched...
The first thing I became aware of is that sometimes he just needs to go off in a grump - as long as he doesn't take it out on me, I just have to let him. There was also a really good tip I got about how to express myself - so instead of saying "you make me feel like crap when you xyz" the counsellor suggested I said "when you do xyz, I feel like crap." Only a small thing but it made him feel less picked on.
I know it sounds obvious but I had got into a rut of banging on about things until he gave me an answer rather than saying something, making sure he understood and then moving on so he could have time to think about it and get back to me when he was ready.
The other reason counselling worked was it gave us a set time to talk about "us" so for the rest of the time, we could just get on with enjoying each other's company. Do you think he would consider counselling?
If your letter is worded sensitively, I am sure it will be a good thing to get him thinking about you and your relationship. You can take the pressure off by explaining you want to give him a letter so he can read it in his own time and pick a time when he is ready to talk about it. You could also say (I did this a lot) that you know you might not have found the right words and that you are not blaming him but you needed a starting point to share how you were feeling.
I do think that if you are unhappy enough to write a letter you do need to do something though. I think DH and I wouldn't have lasted without really talking properly about things.
It might feel really uncomfortable because you don't want to rock the boat but it will be worth it in the end.
Good luck with it - I am sure you are doing the right thing as that kind of unhappiness only festers if you don't deal with it.
And remember that you do deserve to be happy 