Please! i need marriage help!?

I recently got married and our relationship has always had ups and downs. we were always on and off. we have a son together and i am expecting another. About three years ago before before we were offical (for the millionth time) and before we had a son, i started talking to someone else but i ended it once we became official again. He later found out and ever since then, he has accused me of cheating on him.. anyway, he has had trust issues since then. things have gotten alot better these days but its every once in a while he asks, are you sure you arent talking to someone else, are you sure i am the only one etc. i mean, we are married now.. why doesnt he just drop all these insecurites? i married him, i gave my heart to him. When we got married he said those vowels were so powerful to him, why would he forget that MY LOVE IS FOR YOU AND ONLY YOU! HELP!