In reply to Nathan Fletcher:
> Sorry if it came across as being a troll..
Don't worry, you didn't. We're often a bit too quick to assume people are trolling on here, that wasn't you.
It's actually not that unusual that you'll tend to invert while hanging in a sit harness in free space without your feet being against a wall. It varies a bit with body shape, but for most of us standing up straight and wearing a sit harness our centre of gravity will be at least level with the belay loop and possibly a wee bit higher. That's partly why chest harnesses are a thing. (Don't rush out and buy a chest harness!)
Staying upright(ish) actually takes a wee bit of practice and involves engaging your core muscles, some of the posters here might do it so naturally and automatically that they don't even realise they're doing anything. There is a little bit of skill involved in falling off and landing on the rope relatively gracefully, especially when lead climbing. It's not that unusual when a hand pops or something and a fall takes you very much by surprise that you'll end up inverted. Indoors that tends to happen only on relatively steep overhanging routes. Outdoors the risk banging your head on a ledge or something while inverted and/or swinging about during a fall is one of the reasons you might choose to wear a helmet. (Or even indoors, though we mostly don't.)
It's quite important that the waist belt is above your hips and tight enough that you can't slip out of your harness while you're upside down, which is almost certainly the case if the harness fits you at all well.
Hire harnesses at climbing walls are fine, totally trustworthy. Their history is known and recorded and while they see a lot of wear and tear quite honestly they're usually retired in better condition than most us retire our own personal kit. As you may have discovered, buying your own kit does not necessarily solve the problem of struggling to trust it! That said hire shoes are often a bit minging and it's nice to have your own kit if you're going to get any use out of it at all anyway.