You should not have this much issue in WT3 especially seeing as you're level 51. I suspect you either have a really bad build or you're not making use of the codex to at least get the Legendary aspect that syncs well with your main damage dealer attack.

I would suggest taking a look at maxroll builds for example. You don't have to copy them entirely if you don't want to to bit it will at least give you good pointers on how to itemize your build and how to have synergies as well as how to play it. https://maxroll.gg/d4/build-guides?filter[classes][taxonomy]=taxonomies.classes&filter[classes][value]=d4-barbarian

Finally when switching to WT3 usually my first priority is to get sacred weapons with the highest DPS as it increases your attack power by a lot and males the content a lot easier.

Hope you'll be able to pull it off !

I had this exact issue with whirlwind. it seems like it's just nerfed to hell right now, I had such an issue even making it to level 50, let alone finishing the capstone dungeon. looked all over for more builds/guides and I heard mostly the same. I hope it gets fixed soon :(

Whirlwind is rough without either the Bold Chieftan’s aspect or the Marshal Glyph’s secondary effect. Both is obviously preferred, but at least one is almost essential to make the build function in higher tiers. I waited to even attempt switching over to WW until I had enough paragon levels for a fully-powered Marshal Glyph, and it helped a lot.

If the problem mostly about Fury regeneration, seems like you're missing some Legendary Aspect related with it in ring. On barbarian it's tied to shout skills usually

Changing world tiers can be a little challenging. I usually aim for helltides and grit my teeth until i can at least get a couple of sacred/ancestral weapons. The power boost you get helps a lot. When dying, I aim for full defensive top/bottom and when there's a different kind of problem, in your case fury generation, I'd change build altogether.

Run wt3 and go to a stronghold, the levels will only be 2 above you. Run them until you hit 60 so you can equip sacred.