First of all, I am so sorry for your loss, and the fact that your husband felt the need to kill himself. I wish he had sent the note to someone else, you did not need to see him like that.
The good news is; just because you saw that does not mean you will have PTSD. Have you been diagnoised with it? If so, or if not, go ahead and see a grief counsellor, and if needed, a therapist. They can help you work through the issues.
Secondly: embrace your family and friends. They are your support network, and if you let them, they will help you deal with all of this.
Thirdly: Find a support group for people who have lost a loved one to suicide.
Lastly: Dear lady I do not know the cirumstances surrounding your husband's suicide, but in the end it was his discision, and he is responsible for his actions. If you are second guessing yourself asking what if? Then don't. Most likely anything you tried would not have changed his mind, or alltered the outcome.
We, the members of this forum, are here for you, as much as an internet group can be, and most certainly, we will be here to support you, and grieve with you.