ohkay if you go "lets break up" and then tell him how "what we had was great and you're great in all these ways but you'll find someone better" then he's just gonna get more confused and depressed ohkay. he's already "i can't live without her" so you need to tell him how it is. dont make it mushy or cliche or that "it's not you it's me" nonsense. it really is his issue so just tell him why it's not working, tell him that he needs to sort himself out, then tell him once he does someone better will come along to stay, but it's not going to be you and say goodbye and don't look back, dont feel sorry. you have to do it for yourself, and since your already breaking his heart you better thank him for what effort he did put in or what you've learned about yourself and leave. do not coddle him. get it done.
if you want to write a sappy letter then write it, cry about it you don't seem like the one who'll be crying honestly, seal it, and then shove in some folder somewhere in a closet until further notice. and let it be done with.