Past vs past perfect

Past vs past perfect

Hi, I am wondering if past tense and past perfect tense are interchangeable while conveying grammatically correct meanings.
Please help.
J

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About a month ago I went in for a dental check-up. Dr. Morgan [had warned\ warned] me that flossing teeth was as important as brushing them,

I had been meaning to floss regularly, but I [kept putting it off / had been kept putting it off].

So I decided to give this three-week method a try. The first day of flossing wasn't a big deal, but by the fourth day it [had become \ became] an annoying chore.

I [had almost forgotten \ almost forgot] to floss and regretted following this three-week method. But just past the first week, flossing [became\ had become] a bedtime routine. By the end of the three weeks flossing [had become \ became] as natural as brushing my teeth. I was delighted. I [had formed \ formed] a good habit! If I could do it once, I could surely do it again, on a more challenging level.