I've been looking at the wikia and a faq (some puzzles are just... how the hell are you supposed to figure these things out?), but just like the game itself they're all over the place so the only way for me to know what I have to do now would be to read everything until I find something new. I do not have nearly enough patience for that. Here's a screen of my current inventory. Based on what's missing, is there anything you could think of that I should do next ?

Which bosses have you defeated? I see you have the second seal, so I assume you must have beaten Bahamut in Spring in the Sky. If you haven't yet, the next boss to go after is Viy in the Inferno Cavern. Beating Viy unlocks several previously inaccessible locations - most importantly, a door will open at the base of the waterfall on the surface that leads to the Tower of the Goddess.

I've just defeated Ellmac. Before that Bahamut yes, Sakit and obviously Amphisbaena. I haven't found the Ankh in the inferno cavern yet, nor the jewel. I also have access to the Chamber of Extinction, but I can't see anything in there.

IIRC, the jewel for Inferno Cavern is hidden in its backside area, the Tower of Ruin (which has no boss of its own). Or you can use a jewel from any other area, and I recommend doing so if you have one, as beating Viy is essential to unlocking many of the late-game areas.

As for the Ankh, IIRC there is a way to make a pedestal appear at the bottom of that long column of lava that runs alongside that chimney in the Inverno Cavern with all the eyes spouting fire at regular intervals. Placing a weight on the pedestal makes the Ankh appear (and there's something you have to do to get into the room the Ankh is in, too) but I don't remember the details.

And although the chamber of extinction doesn't get lit permanently until later, you can still do a fair bit in there. Note that firing off a flare into one of the black gemstones you find in there will light up the level for a short period of time - just remember to take note of where the gemstone is in each room as you enter it, or you'll have to hunt around for it again.

Thank you.

Unfortunately I'm out of jewels. Guess I'll start by revisiting the chamber with flares then, I didn't go very far the last time since I couldn't see much.

You need the Bronze Mirror to get anywhere now; that reveals the Backside Gates to... well, each backside field such as the Graveyard of the Giants and the Temple of Moonlight. That's your next step in progression.

Yep, I have it now and already checked a few new places. I haven't played since my last post though, I'm taking a break to play some non-headache-inducing games.

Don't blame you. First time's the hardest. Not by all that much, but...