You must understand that penis size of 3.5 to 5.5 inches in erect state is normal. Most importantly, sexual pleasure doesn't depend on the size much. To enjoy sex life, you must learn and focus on techniques of seduction and stimulating the partner. Firstly, you must understand the process of sex and the difference between the male and female sexual responses. Understand the importance of foreplay. Size of penis during sex is not so significant.
Next Steps
For further information and specific suggestions, kindly consult with a sexologist.