The quality and satisfaction of sexual life are influenced by various factors, including personal health status, psychological state, lifestyle, etc. For 40 year old men who are dissatisfied with their sexual life, I cannot simply provide a few suggestions to solve all problems.
The most important thing is to have honest communication with your partner and work together to find solutions. If the situation continues to trouble you and your partner, seeking the help of professional medical or psychological counselors is also a wise choice. They can provide more specific and personalized guidance.