Talonflame

Nope :(

The song I heard sounded very different. I whistled (well, tried to) the melody here https://voca.ro/17IhmkCs2oHu the melody was made up by a piano sound that seemed like a synth or an electric piano (sounded similar to a normal piano, but the sound dragged a bit longer hence why I think it might've been electric or a synth)

There was also this electric bass sound in the background of the melody, which sounded like this https://www.youtube.com/watch?v=_mmj0txco0Q&t=260s (4:15 minute mark)

Listened to a few songs just now, but no. I've updated my OP and have a vocaroo of me whistling the melody of it, which was done via a piano that might've been a synth since the sound dragged on for a bit. The actual melody was slightly slower than how I whistled it. Also I can't whistle very well so I'm sorry.

1. I'm 90% sure it was an 80s song. If it's new, it was very convincing as a classic 80s pop hit.
2. Nothing really matters, nothing ever matters, nothing truly matters. I'm confident about the nothing matters part, some uncertainty of the middle part
3. Might've been a sample
4. I only heard it once, but I was in a shop and didn't listen to the full song before I walked out
5. I can't really whistle, but the part of the melody I remember most went like this https://voca.ro/17IhmkCs2oHu there was this piano that sounded a bit synth that made up much of that melody. There was also an electric bass sound. The melody was slightly slower than my whistled version of it.

Nope

Might've been, or maybe not popular for some reason. I've not heard the song before

Nope. Was definitely sung by a female singer who sounded American.

Sadly not it, I thought it was at first, the vibe and energy is VERY similar. Thanks though, you're helping to narrow things down

I was talking to someone while trying to listen so I only got the lyrics I mentioned. I can remember the melody parts vividly in my head.

Nope, that's not it. If you listen to I Feel For You By Chaka Kahn that's the type of vibe the song I heard gave me, had the same energy, especially as the song gets even more lively towards the end. I even wondered if Chaka Kahn might've sung it. Thanks though

Love Janet Jackson but nope, that's not it. The song sounded very nostalgic 80s and almost sounded heavenly.

Nope. The voice in the song I heard was higher, it was more dance-y, warm and euphoric. It was also faster and Since You’ve Gone sounds a bit slow

Nope