Terms of endearment aren't capitalized. ... A term of endearment isn't interchangeable with a name the same way a nickname is, and terms of endearment aren't capitalized.

Does Darling have a capital letter?

'You' is not capitalized, and nor are other terms referring to the person you're talking to, like 'dear' or 'darling'. As sdgraham says, it may be that you think of 'darling' not as a general term of liking, but as a specific nickname, so you do then; but mostly, no, you leave it lower-case.

Should sweetheart be capitalized?

If you are using a term of endearment as a nick-name then I think it should be capitalised (I'm Australian, hence the 's' instead of 'z'): “I love you too, Sweetheart.” However, if it is not used as a nick-name, then it should not be capitalised: "Jack, permit me to introduce my sweetheart, Heather."