I wrote a very to the point, assertive letter that only talked about me. I need to leave the relationship. either way, it's up to him.
Well you may have started out with a very to the point, assertive stance and but then it became a "it's up to him."
Whether or not your realize it- not making your own choice on leaving and
waiting for him to leave you- is by default, a choice. And you have made that choice- and are willing to live with it. That is not *his* choice- it is yours. You both could go on this way for the next decade and never budge from your present day positions with each other.
If you are not prepared to move out- then do not write letters stating you will. It only becomes an issue of wills from this point on- with emotional wrangling and threats.
If you are *not* strong enough to move out and be on your own- then you are undermining yourself. The inability for you to be on your own only causes more drama in an already faltering marriage and your Husband may use that to keep you from leaving. Either get a mediator together with your Husband and go to counseling or plan accordingly to move out. Do not wait for him to make your decision for you. The letter writing will only increase the dysfunction with two people trying to get their emotional needs met from one another and getting no satisfaction from the interactions.