How can one get rid of feelings of jealousy and doubt towards one's fiancé and his past relationship, especially since his behavior has not changed and he claims to have forgotten that relationship, and is it advisable to confide in him about the matter?

The intrusive thoughts and notions that preoccupy your mind are from the ploys of Satan. Therefore, seek refuge in Allah from him and completely disregard them, for then the matter will turn against him. Remember that this man has become your fiancé or husband, and that girl has become betrothed to someone else because of his engagement to you. This will help you distract yourself from these thoughts. It is not appropriate to bring up this issue with him or ask him about it. You should strive to complete the marriage as much as possible, for prolonging the engagement period or delaying the consummation can be a cause for obstacles that prevent its completion.