Are the feelings of fear and intrusive thoughts that plague me while supplicating for marriage to a specific person, despite my sense of hope and certainty after performing Istikhara, signs of Istikhara or from the whispers of Satan?

Istikharah (seeking guidance from Allah) requires the servant to abandon their own choice for Allah's choice and to entrust the matter to Him. So, after performing istikharah, one proceeds with the matter. If it is good, Allah will facilitate it, and if it is evil, He will divert it. There is no harm in repeating istikharah if the outcome of the matter is not clear.