As others have said, if everyone at the table is having fun, you're doing a good job as DM. (And if you're not sure whether they're really having fun or whether they're just saying that, here's a little test: how prevalent is absenteeism in your games? Do the players make the effort to attend regularly, only missing the game if something important comes up - or due to something they really can't avoid, like work - or do they miss them game quite often, with a succession of lame excuses? If attendence is good, they're having fun.)
As regards the specific weaknesses you listed: lack of rules knowledge, lack of preparation and too much treasure...
Rules knowledge is gained through practice, not reading. Firstly, put down the DMG - there's virtually nothing in there you really need to know. Now, pick up the PHB, and generate some characters. Mix classes and levels, include multiclassing, and take it slow. Make sure you're doing things correctly. Then practice some skill rolls. And a few sample combats. Include the wierd cases - flanking, attacks of opportunity, disarm, sunder, trip, grappling. Have a few spells going off. The more you do this, the easier it will become, as the techniques become second nature. This is the same principle as revising for an exam - you don't want to just know the material, you want to be so comfortable with it that you can use it at will.
(In any event, don't worry about it too much - of all the weaknesses a DM can have, lack of rules knowledge is one of the most minor, and probably the easiest fixed.)
Preparation is something that you're going to have to force yourself to do, assuming of course that you are actually serious about doing it. I recommend starting small: spend some time between sessions dreaming about cool encounters to throw at your party. Then take to writing down the nature of these encounters. Then add the stats behind the encounters. And then start linking them together. Leave a few weeks between each step I've just described, until you're doing the previous steps as a matter of routine. Basically, it's about getting into the habit of preparing.
You can stop giving out too much treasure in one of two ways. One is a 'freebie' that doesn't require you to do anything special, while the other is an immediate quick fix you can use. The quick fix is to use the random treasure tables from the DMG for all encounters. Use random rolls, and don't deviate from the results you get. This will bring the treasures back into line. However, it probably isn't terribly satisfying - you'll want to place specific items, your players will want you to place specific items, and you won't be able to do this without damaging the value of the random system (and using a mostly random system with some fixed elements tends to give the worst of all possible world, IMXP. I recommend avoiding doing this at all costs.)
The good news is that the real solution to the too much treasure issue is to prepare the treasures in advance. Here, you can use the random tables while still assigning fixed items without running into problems, because you'll have time to ensure the substitutions are equal. So, once you start preparing your encounters in advance, you'll find the treasure issue automatically fixes itself.
Of course, you could take the view that you're doing a good job already, so don't need to worry about changing things. Indeed, it's entirely possible that your players like the fact that you're playing fast and loose with the rules, that the games have a free-flowing nature, and their characters are more powerful (due to the greater treasure). If that is the case, fixing any of these 'problems' will actually reduce player enjoyment. So, don't just dive in to fix these things - decide whether you want to fix them first.
