Starting a sentence with the words "I myself have found" is not grammatically incorrect.
From The Cambridge Dictionary:
Reflexive pronouns for emphasis
We can use reflexive pronouns for emphasis:
The director of the company wrote to us himself to apologise for the dreadful service. (or The director of the company himself wrote to us to apologise for the dreadful service.)
We don’t use reflexive pronouns on their own as the subject of a clause, but we can use them with a noun or pronoun to emphasise the subject:
Parents and teachers always pass on to children what they themselves have been told, and this has been going on for hundreds, or even thousands of years.
(This is about halfway down the page.)
While the use of a reflexive pronoun as an intensifier may be grammatically correct, however, excessive use of intensifiers leads to bad writing. In particular, pretentious speakers might put an unnecessary emphasis on their own role in whatever they are describing as a way to make themselves sound more important.
