I say all you need is a fastball and a change up.  Hit your locations and you will be fine.  Learning a cutter at 13 is difficult because the tendency is to twist the wrist (slider).  Wait a few years.  Right now, you want to be a winning pitcher, but I can tell you when you are 20 something you will care less about your success as a 13 year old.  Focus on where you are going to be in 4 or 5 years, your varsity years.  They will be here before you know it.  Location, location, location!  Learn control and stay healthy. 

Because in your first three innings, if you average out, 4 batters per inning, going into your fourth inning, you should be facing the middle and bottom of the order.  Beyond that, the fifth in inning you should be facing the very bottom of the order, thus leaving your reliever to pick up the middle of the order and finish. 

Also, for some reason getting out of an inning with only 10 pitches seems to be a bad thing - it’s not.  If you can sustain yourself with 10 pitches per inning, I would say that you’re doing pretty darn good.  So for five innings at your age, letting someone else take it from there and leaving the field with only 50 pitches, is nothing to sneeze at. 

I should mention that, your level of competition and those who you are facing, is kind of difficult to judge using this medium.  If you’re in a highly contested league, and you have a decent bullpen to draw on, with a reasonable skipper (head coach) then you should focus on what Jerp33 recommended.  Just pace yourself and understand your limits, how healthy you feel going into a schedule and stay strictly to a days-rest regiment. 

At 13, focus on location most importantly.  Hitting the corners of the strike zone is deadly.  After control, focus on velocity and improving your speed.  Once you start to grow and develop a little bit more you will be ready in a couple years to learn a pitch like a curveball. 

Well, then that’s another problem itself.  I can tell you from my personal experience that I’ve tried to throw a cutter but it is a very advanced pitch that is hard to learn and I gave up trying to throw it.  Also, as Jerp said, your tendency will be to twist the wrist and turn the pitch into a slider which I highly advise not to do because a slider, in my opinion, is one of the most dangerous pitches on a pitchers arm. ( I can post links to why I say that if you would like) A general rule when it comes to pitching is that is is better than have 3 or so really good pitches that you can command and have mastered rather than like 6 pitches that are average.  My advice for you is to focus on improving your fastball and changeup and curveball and mastering those pitches.  The question you should be asking if your curve isn’t good, “Why is my curveball struggling?”  or “What can I do to improve my curveball?”  Do you mind me asking why your curveball isn’t good?  No movement?  Lack of control?  Just trying to figure out what is making your curveball not very good.