Here's an interesting question,
"I'm a 14 year old medium pacer. I don't have enough different types of balls, so if the batsman gets his eye in he can read me and whack me around, the balls in my arsenal are: Standard Delivery, Slower Ball, Faster Ball and the Off Cutter (I can also bowl a leg cutter, but I can't control it so I don't use it much).
"And then the thing is I never bowl the same, sometimes I don't take wickets and I don't give away a lot of runs and other times I take loads of wickets and I give away loads of runs. how can I become a more consistent bowler?"
I'm no Sherlock, but I reckon he is giving away the source of his frustration right in the question.
And he is not alone.
This bowler, rightly, wants to be able to bowl accurately and have variations to confuse the well-set batsman.
The problem is that it's difficult to ride 2 horses at once. The more you work on accuracy the less time you have to bowl variations. And vice versa.
So you need a strategy.
First, ask yourself what is more important.
I would say that it's accuracy every time: Even if the ball never deviates from straight and you are military medium. Sure, really really good batsmen (or lucky sloggers) will take you apart, but the average batsman will struggle to do much with 6 balls an over on the spot.
This may be a bit dull but if I see a 14 year old who is even 60% accurate for 4-5 overs I would have him in my side in a flash. Most kids - according to PitchVision Interactive (the online comparison tool for cricketers worldwide) - are below 50% accuracy on length and even the very best are under 65%
And there is a clear correlation between the number of balls you bowl that are a good length, and your strike rate. That's just a fact.
So, Get your accuracy of a good length ball on off stump up as high as possible.
Lay down a target or use PitchVision and bowl at it until you can hit it regularly.
Then bowl at it some more because you can never achieve perfection.
Plain, simple, dull and very effective.