Having a lover may improve your sex life and your self esteem, although the sex might not be as good as the sex in your imagination, and if it is casual arrangement you may not feel any more loved. But it will almost certainly increase your dissatisfaction with your husband whether it is kept secret or not. If your husband becomes aware of your affair then that will not help improve your relationship and he may divorce you for your infidelity. Is it possible that the new man in your life represents a transition towards eventual separation and escape from an unhappy marriage? If so, take care as married lovers in casual affairs rarely leave their wives..
I once had this dilemma many years ago, decided that I couldn't live with the guilt and my responsibilities towards others were too great at the time, so I didn't have the affair, but the decision not to was an incredibly difficult one. Had I not had responsibilities towards my children at the time, I probably would have had the affair, as the magnetic attraction was almost overwhelming. I did eventually leave my husband many years later, but not because of anyone else. Ironically I think my husband found that harder to come to terms with rather than if had I left him years earlier for somebody else, as he had no one else to blame for the breakdown of the marriage.
Whatever you decide there may be unforeseen consequences.