Don't know if you found it already, but: http://www.mangago.me/read-manga/goodbye_lilac/ (ch 5).
Omg thanks!! The line in the manga is a bit different from what I said I can't believe you found it! Lol thanks really!
I knew I'd seen it somewhere before, so had kept a tab open with your question, but I couldn't remember where. And then I just happened to be rereading that chapter (looking for something else entirely), and there it was! :) Glad it was the one you were looking for :)
Sorry if it's too vague but I'm looking for a yaoi manga where the seme tells the uke something along the lines of "I can't promise to keep you happy but I'll help carry half of all your burdens." It was really sweet and I seriously can't remember anything else that line really stuck with me and it's been bugging me since I couldn't find it.