My cupcakes always come out heavy if I don't use a box mix, what am I doing wrong?????

overmixing? too less eggs?

it is possible you could be over mixing your batter, It could just be the recipe your using its good for cuppies, its worth experimenting till you find something that works

its all in the batter... i found there are a few really important factors....

1. Make sure you are creaming the butter and sugar really well.. this put air into it...
2. beat in your eggs into the creamed butter mixture really well one at a time..
3. sieve your flours and do it from above the bowl to allow for more air...
and
4. dont beat the flours into the wet mix.. make sure you take it out of the beater, or if you are hand beating, put down that beater, and just carefully and quietly fold the dry ingredients into the wet ones ... this will mean you retain maximum air in your cupcakes and they will come out beautifully light and fluffy... mix until the wet and dry ingredients and only just mixed in... and make sure youre using baking powder!

Hopefully that works for you... if it still doesnt, change your recipe, it might not be you at fault but the actual recipe you are trying to use, I have found this same thing as well, so its just about experimenting! GOOD LUCK!