Okay, let me ask you first, are you trying to be a competitive type player, or are you just wanting to be the king of the hill amongst your friends? It’s okay if you aren’t looking to be a tourney player, but depending on how far down the rabbit hole you are willing to go there are varying degrees of what you can do.
If you are just looking to beat the CPU, it’s way different than beating a person. People are generally the tougher challenge, so if you are looking to beat human opponents, it takes some gradual steps up, but you can get it.
It sounds like from your experience though, you may be confused by thinking that learning long intricate combos is a key to victory, but that’s far from the truth. If you can’t hit your opponent to get that combo going, then learning those combos is meaningless. You need to see where and how you can hit your opponent, and from that first hit, go the next, and on and on for max damage. That means learning to punish their mistakes, and forcing them to make mistakes.
The best advice I can give you is this. Focus less on the combos, some of the challenge mode stuff is impractical anyway, they are mostly challenges for players more experienced. Instead, start playing real people, and see what they do in certain situations. The more you play, the more you learn, and in the end, experience is the real winner. And don’t start with ranked, it’s not that forgiving. Instead, invite friends to play, or stick with endless lobby or something unranked, and try to find people who are willing to talk and listen to you.
Also, know when doing combos, the timing of your buttons presses is REALLY important, and mashing stuff out isn’t a good idea. If you are doing stuff too fast or too slow you may end up dropping a combo or something. Learning the timing to button presses and combos does involve a little muscle memory, so it takes some practice.
Finally, don’t focus so hard on winning, but on learning. We all become frustrated, but skill takes time to accumulate. A man learns more from one defeat than a thousand victories.