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I planted a mixed pack of flower seeds, and this one has grown but has very thin stems and keeps falling over - it's only standing at the moment because I've strategically leant the two stems on each other!

What is it, and why can't it stand up?

It looks very shady in the picture and right next to a wall. Does it get enough sun? One reason for plants growing very tall and spindly is that they aren't getting enough sun, so they stretch themselves up tall trying to find some.

I have some flowers that do that. The stems are not strong enough to hold up the flowers. Tropicana roses did that so I actually planted a different rose when it came time for it to be cycled out. When I plant flowers with weak stems together they can help hold each other up. Otherwise they need plant supports.

Thank you for your responses! I took this photo in the late afternoon - the pot is actually on the north side of my house so it gets plenty of sun! I might try supporting it until the other plants around it get bigger and can help hold it up.

I had to stop and think a bit. Where I am the north side of the house is not plenty of sun, but that stuff is all reversed in the southern hemisphere!

Another thing than can sometimes make plants spindly is over fertilization, making them grow too fast.

Hahaha yes that's right! I always forget to make it clear - most people on this forum are in America so it does get confusing
I haven't fertilised it at all. I'll keep an eye on how the others grow and we'll see what happens!