I was envious forof those who were left-handed when I was atin school, because I wanted to be a peoplsomeone who was different from others.

Instead of at, I think in school sounds more natural for this particle of time.

But I think now that being left-handed is pretty hard to do something because there are less left-handed products.

I was envious ofor left-handed people when I was atin school, because I wanted to be a peoplsomeone who was different from others.

Use "in school" to refer to a stage in your life, and "at school" to refer to your physical location. Since you're describing your feelings while you were a student and not just how you felt while in the school building, "in school" is correct. Some examples:
"I felt anxious when I was at school, but at home I felt confident."
vs "I felt anxious when I was in school, but after graduating I became confident."

But now I think now that being left-handed ismakes it pretty hard to do somethings because there are lessfewer left-handed products.

1. (Optional) Adverbs of time such as "now" and "soon" can be placed in different locations of a sentence depending on what you are emphasizing. In this case, I would place it before your verb ("now I think") because it more clearly emphasizes that we've shifted to your current-day opinion.
2. "Left-handed" is an adjective ("I am left-handed") or adverb ("I play guitar left-handed") rather than a noun. In this sentence, use "being left-handed" or "left-handedness" to refer to the state of being left-hand dominant.
3. Less vs Fewer: Many native English speakers will mix these two words up. "Fewer" is used for things that can be counted, and "less" is used for things that have an amount you can measure. Examples: "I have fewer coins now" vs "I have less money now"

I was envious ofor left-handed people when I was at school, because I wanted to be a peoplerson who was different from others.

But I think now that left-handed is pretty hard to do somethingnow think it must be hard to do things if you are left-handed, because there are less left-handed products available.

If you add 'it must be,' it shows that you do not have the experience of being left-handed, and you are guessing what the experience might be like ˙ᵕ˙