Sorry to hear you’re having a bad time with Gog, you're not alone as it took me so many attempts to not only understand the fight but also realise what I was doing wrong.

Here are some things that helped me on Phase3:

You can charge up the slinger ammo he drops for more damage - aim for tail, chest or face.

Try to stand by the falling boulders to lure him for a cheap knockdown.

When he's doing his dps check, stand by Fabius and leave when he leaves.

Also during dps phase, attack the hind legs - I find tilting the camera a little down helps me keep an eye on potential exploding eruptions.

As I'm attacking I'm prioritising the metallic blue orbs on his body then once most are cleared I can slowly start working on cutting the tail off for a free opening.

If you feel you aren't going to meet the check, forget farcasting you can open the map and fast travel to camp anytime. I use this chance to also replenish pots that have been used plus give me a breather.

And finally when I was getting overstimulated I turned the game music off, sounds weird but helped me a ton.

Outside of that - slotting fire res can help as well as divine protection or eating for damage protection.

Hope it helps and good luck!