Am i trying to go too big?

Take a week off. About every 10 to 12 weeks I take a week off to give my body a chance to rest. Then when you start back use less weight the first week.Back off a little on the weight when you start getting sore tendions. I get it in the elbow bad. When this starts I back off on the weight and switch to more machines for a week or two.

Try interval training, this is where you work out in 7-9 day cycles. What you want to do is start out easy at the begining of the cycle and then work up to a hard work out at the end. Then take a day off and start all over again.
Also try less weight and more reps. If you are having sore tendons, you need to stretch. Try stretching both before and after lifting, or at the end of the day before you go to bed.
Your young don't get in a hurry, stength will come with time, you need to be more worried about flexiblity and cardio conditioning. Very few sports take pure brute muscle, but flexiblity and good wind are need for all sports.
You also need to be careful that you are not over building any small one area of muscle groups, you need balance. I would suggest finding a trainer to make sure you are doing the correct exercises. If you are still in school you might have access to one if you ask the right people.

Take a rest! At 15, you are still doing a lot of growing. Take your time and don't get hurt. You have only got one body. Take care of it.