I haven’t been cleverforgetful since my childhood. I have been forgetful since my childhood.

This sentence is 100% technically correct, but "clever" sounds odd phrased like this. Maybe "forgetful" in order to keep the sentence mostly the same? Otherwise, "I've never been clever" would be more natural.

This is especially true when Isince I've become old now. This is especially true since I've become old.

1) "when" sounds like it hasn't happened yet. Try "since" 2) deleted "now" because it sounds more natural without it

I like studying Chinese, but I am very sad thatbecause it seems my Chinese cannot be improved at all. I like studying Chinese, but I am very sad because it seems my Chinese cannot be improved at all.