In spite of having your relationship, he developed an affair with other woman. Therefore, one thing is very certain that he took you and your love for granted. If he loved you then how could he developed an affair? Alternatively, that time there was no love and now that he has developed it? People does not change overnight so do not think that he is changed now; it is your love which makes you think this way. You should not allow any one to play with your emotion like this. Try to take control over your emotion and come out of your attachment. This will be better for you on long run.