best product for curl definition and moisturizing that won't weigh down hair

when I was younger I had very curly hair (as you can see in the pictures). Now, I sometimes have curly hair and sometimes have stringy/wavy hair. I never know how my hair is going to come out each day. I was wondering why that is. I love when my hair is curly and hate when it gets stringy and looks very dried out. Is there a way that I could get my hair to stay consistently curly each day? Any product recommendations?