A good starting point might be to analyse the primary issue that you have trouble with: heart triad types (2,3,4) have self-image issues. Head types (5,6,7) struggle mainly with anxiety, and for gut types (1,8,9), their problem is anger and autonomy. Think which problem has been most prominent, because that will define your core type.
4 is unlikely to be core for an ISTP. 5 most likely, but 8s are common also.