There are actually a lot of good resources for getting better at fighting games. I know Ultrachen (Ultra david and old man James Chen, two of the more renowned American commentators) have pretty comprehensive guides for most games that release as well as youtube channels like Vesper arcade
http://www.twitch.tv/ultrachentv/profile/past_broadcasts if you go to the past broadcasts here, you can find a lot of recent MKX information, including just a very basic summary of each character, as well as more advanced stuff (such as run downs on mortal kombat option selects.....don't worry about that for now)
there was also a recent tournament with very high level play for this stage in mortal kombat X's life, which you can find under past broadcasts here
http://www.twitch.tv/nycfurby
Like most things though, getting good at fighting games is a lot about knowing how to learn. You need to be able to identify the rules and concepts of the game in order to identify what you are doing wrong. For example, what you say is unavoidable high low mixups some characters can't even do (most can though, high low in this game is actually pretty crazy, just gotta avoid the situation) and when approaching others and feeling like they have greater priority, well that is probably just them having better footsies (weaving in and out of the range of your attacks and challenging with their own once they feel that you are in range of theirs, sort of), or you not knowing a safe approach (such as you rarely want to dash forward in MKX, because running foward, although it uses stamina and therefore prevents you from breaking, is cancelable to any button including attacks and more importantly the block button)
for combos, I don't know if this helps but Mortal Kombat X combo system is so that as long as you input the next move within the start up of the move you did prior, it will come out. No matter what (not withstanding option selects) Even on whiff. Most combos in mortal kombat are generally easier to do because of this, but like 3d fighters Mortal Kombat requires a solid amount of memorization. When it comes to combos, it is really just practice. There are always basic bread and butters for each character that you want to know, start with just focusing and applying a few of those (I.E. combos off of high an combos off of low), before trying to incorporate more. Don't practice combos that are only possible off a slow start up mid for example, because something like that will only be needed for hard punishes even if it looks cool or does the most damage
As a general rule though, you want to focus more on how players are putting themselves in situations to land their combo, the benefits of certain combos as opposed to others, and what combos work in what situations (confirms, becomes one of the more important aspects later on). Once you understand these things you will be able to better understand what your opponent is looking for, and use that to your advantage (by reading them)
Mortal Kombat X does have terrible netcode though, so footsies and reaction times often will give way to bad footsies and predictions as opposed to reactions, but I digress.
I know the learning curve is some what steep, but very few things are more rewarding. Try to realize that frustration will only leaving you banging your head on the wall trying the same things over and over, and that will stunt your growth. I could write an essay of advice but there are better sources than me for that
Regardless of what some people said, fundamentals such as spacing and footsies transition to almost any fighting game, even learning combos will be easier to some degree as well.
Good luck getting gud