The verb to persevere means to continue doing something despite difficulties or obstacles. The verb has different forms depending on the tense and aspect of the sentence.

The base form of the verb is persevere, and the past tense and past participle forms of the verb are persevered.

In the sentence, the phrase if only introduces a conditional clause that expresses a regret or a wish about something that did not happen in the past.

The main clause uses the modal verb could, which indicates a possibility or an ability that was not realized in the past.

Therefore, the correct tense to use in the conditional clause is the past perfect, which is formed by using had and the past participle of the verb.

The past perfect is used to describe an action that did not happen before another action or time in the past.

Therefore, the correct way to express that we did not continue doing something despite difficulties or obstacles in the past is to use had persevered, which is the past perfect form of persevere.