The answer is in front of you

Question

I'm myself most of the time, but a third I am not
I'll trick you, my boy, like an insect you caught
I start, I end, but I'm not in my middle
I'm here right now, all throughout this riddle

Who am I?

Answer 1

You are the word:

CONSONANT!

I'm myself most of the time, but a third I am not

The word CONSONANT has 6 consonants (the majority of the word) and 3 vowels (a third of the word).

I'll trick you, my boy, like an insect you caught

CON + SON + ANT

I start, I end, but I'm not in my middle

The first letter ('start') is a consonant (C), as is the last letter ('end'): T. But the middle letter is 'O', a vowel.

I'm here right now, all throughout this riddle

There's an awful lot of consonants in the text of this riddle!

As for the title:

Not only are there many consonants on the screen in front of you right now, but the 'front of YOU' is a consonant (Y), and in the English alphabet there's a consonant (T) directly in front of 'U'!

Answer 2

Alternate (and perhaps lame) solution:

myself - i.e. “I”, ego, ...

I'm myself most of the time, but a third I am not

I'm myself, I feel, I think, I have qualia etc... But one third of time we sleep.

I'll trick you, my boy, like an insect you caught

That old tricky grammar question, where formally you'd expect “I” as a nominative subject complement in sentences like “It's I”, although the standard English uses the objective case: “It's me” (considered often incorrect from the prescriptivists' point of view).

I start, I end, but I'm not in my middle

The word “I” starts with the letter I, ends with the letter I, but since it is a one letter word, it has no middle.

I'm here right now, all throughout this riddle

Each of the lines in the riddle starts with an “I”