up to you to decide how your current relationship is going, but yes, dwelling on past relationships may be an early sign that you are having some doubts about your present one. Or it may not, it may just be as 4getmenot says and scars don't vanish in a hurry. So have you got any suppressed doubts about your new bf? Can you put your finger on what exactly triggers memories of the last ones? (Particular foods or songs maybe?) This is basically all in your head, so only you can really sort it.