Okay, let me reassure you: You are NOT an idiot.
The game is unlike nearly any other shooter on the market.
The problem that I’m seeing from your post, is that you’re starting with the game, trying to learn Melee tanking. hands down, that is the hardest position to play. Melee DPS can pretty much bail on a fight, but tanks get all the attention once they’re exposed and vulnerable.
Caldarius is equally tricky, as would be Mellka. They’re both hybrids, able to dance in and out of a fight fast. But it’s knowing WHEN, when to make the big entrance or exit, that makes and breaks players.
I’m going to break down some suggestions for other characters to try, based on what I think you want. It’ll be a long list, but it should help.
At this point, I would recommend NOT going Melee tanks. It’s far harder, and less forgiving of mistakes than being a ranged one. Now, if you want to learn Melee DPS, El Dragon is right up your alley. Save his Clothesline, not for a new attack, but for the single most FRUSTRATING ESCAPE MECHANISM IN THE GAME!!! Sorry.
if you want to get used to tanking, I’d recommend ISIC or Monty to start with. Both are very robust tanks, that can deal with being hit, and deal out a lot of hits on their own. You need to learn how to initiate a fight, and sustain yourself for a bit, either until the enemy runs, or your healer gets to you. At that point, decide whether you want to back off or try and get the kill. DON’T always try and get the kill. Your job as a tank is to start the fight on good terms. Let a DPS get the kill, live to get the assist, and you’ll have a more prepared team for the next enemy.
As for alternate DPS characters, you could try someone like Oscar Mike. Don’t let the CoD influences fool you; he’s the best pusher and one of the best ranged DPS’s in game. At level 2, pick up the Fragcendiary Grenade Mutation, and just watch the world burn. Another character I would recommend as a learning tool, is Whiskey Foxtrot. Again, the gun shouldn’t delude you. He’s a cross between Oscar Mike and Marquis. He doesn’t get Marquis’ big crits and punishing AoE effects, but he can be a good tool in learning how to reverse a fight and capitalize on it. I would personally recommend shying away from Orendi and Benedict at THIS point. You need the fundamentals first. They both have a very…unique playstyle. Some people will grasp it instantly. I’m two months old with BB, and I’m STILL trying to work out how benedict’s DPS works. 
One last thing: keep running the PvE content. it’s great for leveling your player rank, good for leveling specific characters, and is the best way to get more gear items. Don’t focus on Legendaries yet. Just get lots of blue and purple loot, and try to avoid keeping anything with red text; that’s a negative mod, that affects any characters who equip that item with shards. You want more items, because it will give you more build options, and let you take different roles for the same characters(A DPS Galilea, a soft-tank Rath with so much lifesteal that his vampire non-festish will be a flat out lie…) Treat the PvE as a laboratory of sorts. Go there to try out different Helix combinations, and see how they interact. It’ll serve you a lot better than trying to trot out an untested character into PvP. Once you have your build idealized(remember, it worked against bots. SOME things won’t work against players) take it to PvP and see where you stand.