I think you must have been very hurt or cheated on by someone you loved and trusted and that is why you are feeling this way without him giving you possibly any reason to think something might be up. It will take time to get past that feeling, but would pass quicker if you maybe talk to him about it and later on and how pass relationships affected you? Try not to think the worse and push a good one away.
“Accept — then act. Whatever the present moment contains, accept it as if you had chosen it. Always work with it, not against it. Make it your friend and ally, not your enemy. This will miraculously transform your whole life.”
― Eckhart Tolle