So here's what I've found (MINOR SPOILERS):
If you say that you love him the very first time he asks you (Day 25), then you just have to play by ear until the end. Choose the options that lead to more intimacy with Harold, like duh make him a cake instead of a sonnet.
In my opinion, saying that you love him on Day 27 instead of Day 25 offers a better story and makes more sense for the character, but really it is up to you.
Anything past Day 27 or 25 (if you don't tell him you love him on those days) will end up with the bad ending regardless if you choose the best options.
I had to force myself to see the neutral and bad endings but now you, and anyone else who reads this doesn't have to.
(Edit) No edit nevermind