There's a separate MMR associated with each game mode, the downside to this being that it's difficult to find where you "rightfully belong" in each category, especially in Ranked where elo-boosting, smurfing, and duo-queue-carrying are prevalent.
The main thing from my experience is the mentality of players. In Ranked, they will generally pick their best champion - someone who they are comfortable with and can carry the game at any point in time rarely having bad matchups. In Normals, it's a large practice mode for getting better and, well, practicing picks that you would bring into Ranked, or having fun. It's not uncommon to see Kindred being brought into lane while Vayne goes and jungles in Normals, however this rarely happens in Ranked, where people's MMR/LP/Rank is on the line.
You'll notice in Ranked that Pings are usually answered much better and Wards/Vision Coverage is much more substantial for your team. This is often times from comfortability of a pick - they are able to assess the situation and understand, both mechanically and mentally, what's going on and how they should play it as they've seen it a million times before. Let's say you main Gangplank and know he's very weak early game and have been punished for it many times before. Placing a ward in the bush at 3:00 might protect you from us filthy junglers a handful of times, and may end up engraved into your memory to do so nearly every match. In Normals, an inexperienced Gangplank may not understand his powerspikes and how slow his Barrels decay early-game, and as such may overestimate his own power or underestimate the gank potential due to "loligotoranges." That's often times why you'll see lots of threads on forums complaining about "DON'T FIRST TIME A CHAMPION IN RANKED" because it's costly to the team and overall affects people's rating - although this is just a game, Ranked is the creme de la creme here and some people take it very seriously. Some aspire to be professional gamers some day and it's very difficult to "carry" a game and climb up the ladder if an enemy champion is busy farming your team like minions.
Don't get so down on yourself for your performance, you seem like a nice person and it's very refreshing to see someone take an introspective position on this rather than flaming their teammates. Keep it up, I'm sure you'll climb to where you want to be! Check out your replays in the client and figure out what you could have done to make a better outcome of each situation, as well as look at your builds compared to what the enemy team has to offer. Even if you go 0/10 on a champion you're comfortable with think of it as a learning experience and try to grab as much knowledge of the situation that you can. Best of luck to you, I look forward to seeing you climb and reading a post from you in the future of "How I got to Challenger." You have a great mentality for a team game!
