Sometimes, someone who loves you wonders why he was so lucky to have found you and are afraid of losing you that they feel less than other men. It does have to do with your life experience. How our lives have been lived determines who we are today and how we feel about different things including a major love feeling for a special woman.
My boyfriend has some of these same feelings and I tell him when he feels this way to tell me and we talk until he feels safe and secure in our love. The frequency of these times is getting further and further apart. I am always patient with his feelings and reassure him I am not looking for another man in my life and if I ever did, I would tell him up front and listen to his concerns. When a man is treated badly in the past whether by family or lovers, he carries the pain and hurt around for his whole life. When he finds a women who truly loves him for who he is, then he is sure she will go away at some point. We can reassure him as needed until he is secure enough to relax and just enjoy our love for him.
I agree with mono about therapy and its helpfulness in learning to deal with these feelings. Some people can do and some cant. Just talk to your lover and you can learn to feel better about your love and its preciousness and protecting it. Take it one day at a time and you will last a long time.