What's a game that can have me addicted hardcore for like a month?
I've already played many of them including the soulsborne types and WoW.
What about Guild Wars 2? I was a fan of the first one.
I'm basically working 5-10 hours/week and the free time is killing me of boredom.
Depends on what you want out of a mmo. As with any mmo end game is probably where you get the most fun.
There's a ton of content in gw2, not sure how far you would get in month, depending if you buy the expansion or use a f2p account, but if you're starting from scratch without help might take you a 3-5 days to level to 80? core levels tend to be a bit vanilla/boring, although if you get the expansions you can get the raptor mount early on. Then you get into the end game content of world vs world, fractals(dungeons), strikes, raids, world bosses, full map meta events, conquest pvp you can join from level 1(everyone is level 80 in there and you don't need pve gear), 5 expansions worth of more content, 5 living story seasons.
There isn't a gear threadmill like wow, leveling you will get greens and blues and yellow rares, but 80 you get exotics and ascended, exotics are easy to get, ascended a little more effort from certain places, but the stat difference isn't very much. Legendaries are same level as ascended for stats, they're mostly for looks and convenience to change stats whenever but require a lot of materials, and it's account bound so all your characters of the same armor class get access to it (the light set you can equip on your elementalist, mesmer, necromancer for example, trinkets like rings can be equipped on all characters but you only need to get it once for one slot). Raiding you would do for the raid legendary set which is animated when you go into combat, wvw and spvp and overworld have their own legendary sets too. Would take a few months to get those though.
Combat is pretty free flowing, you don't stand in place to cast skills (well other than a couple animation locked ones), it's one of the best parts of gw2, although class balance is something else in some cases. Wvw can be 70v70v70 combat during NA prime time, max squad size is 50 so you might find 1-3 groups of 40-50 players running around in the maps at a time, but there's room to roam and do small group stuff. Although the game mode isn't as rewarding as pve, and it's been slowly degrading with it's recent developments, it's turned from siege warfare more into team deathmatch between zergs.