The meaning at the end is simple, she just broke her promise cause she said her girlfriend was never gonna feel lonely but at the end she did it(unconsciously maybe) that's why she was looking at the fireworks with the same feelings from before

I always thought this had a real ominous ending... until you realise she's upset, went to bed and sulk

Because it's a one-shot it feels bad, with the ending being very "final" hence why the default assumption is that she either died or left. But realistically speaking there are plenty of ways for them to make up, fortunately. We're just left to imagine them~

Although I'm not going to pretend that I won't immediately go read something that shows a happy couple.

I don't really see this as sad, more incomplete.

I don't see any evidence she killed herself. But it's possible she might have left her. Way I figure what actually happens is she gets back from her work trip finds her girlfriend has left her. They both discovered that they've been hurting each other and promised to be more honest and communicate with each other.

Like I said not a sad ending just incomplete.