If there are mobs same level as you and you cant kill them, set the difficulty back. Its a l2p issue and not the game being hard.
You need to balance your character with equal amounts of defensive and offensive passives and abilities. Chosing only 1 path will hinder you.
You get the mount after completing ACT 3 and it work like this: the closer to the mount you move your mouse cursor the slower it will ride and vice versa.
Here is an interactive map of every single quest/shrine whatever you need.
h ttps://d4builds.gg/map/?fbclid=IwAR1stbTauqlJdND6adrwe3pLs-a7WxfKTrW_g02cKeDOYNKZ6nyN_XL9bEU
Remember to upgrade your items at the blacksmith and jewelry vendors, but no further then lvl 3, save the exotic materials for when you start getting high end loot. And remember to do nightmare dungeons on regular basis to upgrade your rare glyphs for your paragon tree.
And finally dont set your mind on only wanting to play 1 type of build from start til finish, it will hinder you. Your build should change whenever you get a powerful legendary that have certain skill bonuses.