*snip*
Secondly, I have no clue about 5e rules. So I have no idea why my hit chance was so low, or why hit chances generally seem to change at random. To be honest, I just want to play the game. So the fact that I felt compelled to check this out, because it is never explained, and because the swings are so massive, suggests to me that something is broken here.
*snip*
Honestly, that sums up the problem your experiencing. You don't know the rules of 5e.
The reason why you're experiencing such a drastic change is because there is such a big swing going from Disadvantage (roll 2 d20 dice, take the lowest result) and Advantage (roll 2 d20 dice, take the highest result):
When you are attacking, here is the math that's going:
Roll 1 d20 dice. Take that result, and add your Proficiency bonus (+2 at this level) and your Ability modifier (Strength for most melee attacks, Dexterity for most ranged attacks, so usually +3 at this level). This, the math can be seen as 1d20+5. Compare the total result to the enemies AC. If you match or exceed the enemy AC, you hit.
So that means, for a basic attack, you have a chance to get any result between 6 (1 on the d20 +5 from your Proficiency and Ability modifiers) - 25 (20 on the d20 +5. 6-25 are your possible results (ignoring crit miss/hit for now). As you can see, the overwhelming majority of your chance to hit something depends on what you roll on the d20 dice!
Now, what happens if you roll the d20 *twice*? If you're taking the lowest, the math works out that Disadvantage (again, on average) is the net effect of having -5 to your total result. That's huge! It's literally the same as removing your entire static modifier (the +5 from above). The math is the same for Advantage, now you have +5 to your total result, again that's huge!
So compare the results. A normal roll is 1d20+5. A roll with Disadvantage is 1d20+0. A roll with Advantage is 1d20+10. The difference between attacking with Advantage versus Disadvantage is literally 50% of your possible roll (+0 versus +10)!! If the Enemy has an AC of 10, you'll almost never miss with Advantage but you'll miss 50% of the time with Disadvantage.
