@karasutengu "I'd felt" = "I had felt", meaning the verb is in the past perfect tense. Thus, the "felt" here is not a simple past tense but rather a past participle. The same is true of "opened". "I'd opened" = "I had opened". The "had" there also extends to "seen" because the "and" is joining the two verbs of the compound predicate. The "I" here is performing both the actions of "had opened" and "had seen", so the two verbs must be in the same tense, past perfect.
"I had opened and had seen" isn't technically incorrect, but it sounds stiff and awkward, which is why English allows the second "had" to be dropped!