Had striven.
The present perfect continuous is formed with have/has + been + present participle (-ing form of a verb).I have been strivingWe have been strivingYou have been strivingHe/she has been strivingThey have been striving
The past tense of "strive" is "strove."
The past tense for "strive" would be "strived" or "strove".
Existed is the past tense. The past perfect tense is had existed.
The past tense is bled, and the past perfect tense is had bled.
The past tense of sit is sat; the past perfect tense is had sat.
The past perfect tense of hope is had hoped.
The past perfect tense is had set.
The past perfect tense is had submitted.
The past perfect tense is "had confronted".
The past perfect tense is had rung.
The past perfect tense is had burned.