There is no two ways about this, you have to hit the gym, or start some work out routine to get in shape and build yourself up. However, the most important point to focus on is diet, eating enough too put on weight is the most important part of body building and strength gaining. Improving your upper body strength just a little bit will improve your strength massively overall and your athleticism overall also.
This will come over time, but to really see large improvements in it, you need to get your head up. When you are running from point A to B off the ball, you do not need to look in the direction you are running 100% of the time. You don't even need to look in that direction for 50% of the time unless it is crowded. So use this to your benefit, and always have your eyes scanning around to see how things are unfolding around you, and give you a general idea of where everyone is, like a mental satnav.
The next large improvement you can make is to stop ball watching.
When your team mate or the opposition has the ball, don't just stand there like a Neanderthal watching the ball, keep looking around for space you may run into or opposition who are making runs for you to track.
Finally, when you are on the ball, don't be a heads down eyes in player. When the ball is controlled, get your head up! when you are just jogging with the ball at your feet slowly, there is no need to watch it. So have your head up looking for potential passes as your team mates move into space.
Another one most players fall into the trap of is watching the ball as they receive it. If your eyes are down watching the ball, they are not watching to see if you are been closed down, if you have time to turn, if there is a good one touch pass on. If you cannot see these things happening, there is no way you can have a chance to react to them effectively. When the ball is coming towards you have your head up looking around, and then as it comes near watch it and focus on your control, before looking up again to kept watch.
A good example to follow is a player like Xavi or Iniesta, they exhibit all of these qualities 100% of the time they are on a football pitch.
There are a variety of things that can cause inconsistency. The usual suspects however, are under-confidence and improper preparation.
Under confidence is pretty self explanatory. You lack self belief and therefore cannot do things you are capable of. You will never achieve consistency if you are limiting your potential inside the confines of your skull.
Not eating enough. Not sleeping enough. Getting stressed out from not doing your homework on time. These are examples of things that may seem relatively small, but will massively impact your game.
Not eating breakfast results in your body going into a sort of starvation survival mode. This is what causes you to feel hunger disappear after a period of a few hours. Your body is actually going into a low activity hibernation mode to conserve food until your next meal. This is bad because it means you are trying to play a physically demanding sport, which requires large quantities of energy, on an empty tank. Avoid this like the plague. Same goes for dehydration, which is even more performance inhibiting.
Not sleeping enough causes your reaction times to increase (as in they get longer and longer which is bad) and your overall thinking capacity to drop. Sleep at least 8 hours a night if you want to be in good shape for the game.
Not doing homework is a strange one, but little things that get on your mind increase stress levels, and take away focus from the game at the same time. This is bad when you need to take part in a sport that requires 100% focus and 100% relaxed frame of mind.