Pitch letters and pitches appear impossible things to get right - I've heard lots of people say they find writing the pitch harder than writing a novel. But there is a different way to look at them which should help.
Let's suppose your submission to an agent has worked and they've taken you and your book on. Now what? They sell it to a publisher - that's obvious. But how do they sell it?
They use whatever you said in your pitch letter as the basis for their pitch. So, when you're writing your pitch, think of someone else using those words to sell your book to another person and then ask yourself if you would buy a book on that basis.
A mother of three from Portsmouth - would you buy a book on that basis?
An accountant with an interest in railways - would you buy a book on that basis?
A novel that's taken ten years to write - would you buy a book on that basis?
A novel that's based on a dream someone had last summer - would you buy a book on that basis?
All these may be true and interesting to you, but I don't think they would sell a book to a stranger. What about...
She's won several short story competitions. He's had articles published before. The novel's a Cold War thriller/a time slip romance/a journey of self discovery...
Keep asking yourself - would you part with your hard earned cash on that basis? If the answer is no, then don't write it in the pitch.