Congratulations for taking the plunge! It's scary but it's something we all had to go through in order to improve.
I think you've got a good starting base but both the letter and synopsis can be improved. They are both tricky and can take a while being tweak until they are just right.
Something in both the letter and synopsis is that every word needs to earn its place and anything superfluous or that doesn't contribute should be cut.
am writing to seek representation for my first novel, Reality Filter, REALITY FILTER (title are either in italics or all caps, just choose whatever works and be consistent) a 90,500 word dark comic urban (dark comic is not really a genre but from the info in your letter I would guess it's urban fantasy) fantasy, complete at 91,000 words (just round up the word count) set in modern day West Yorkshire.
An assistant psychologist discovers that magic is real, which makes her job even more difficult. What makes it extra hard, is battling the same ancient darkness as one of her patients.
Kelly Donaldson leads a normal, sensible life. After losing several things that she has only just put down, she decides to investigate. She discovers that magic is real, and the world as we know it has been subject to a Reality Filter. Kelly has just smashed her own filter to bits and there is no going back. She can now see and hear things that are Really There, including three imps, who’ve decided to befriend her. This is extremely distracting, especially when she is trying to support people who see and hear things that AREN’T really there, or so she thinks.
Kelly’s discovery takes a dangerous turn when she learns that one of her patients is plagued by an ancient being made of shadow, and it is up to her to do something about it. She seeks help from imps and learns that she must risk everything to save her patients’ life; her job, her mental health, and her sensible life.
That blurb comes across as a little flat and matter of fact. The normal person discovers there's a hidden magical is a known trope of the genre. Bear in mind agents receive between 60 to 100 submissions a week so your letter has to stand out and needs to show why your story is different and the one they should be reading. My advice would be to select about 10 to 15 books in the same genre as yours and study their back cover blurb.
Also you are missing your comp titles. Agents would expect about 2/3 comparative titles ideally published within the last 5 years and bonus points if any are a debut.
I am in my mid-thirties, living in Yorkshire with my husband and three cats,
and whilst I’ve been writing all my life, this is my first novel. I look forward to writing more in this series.
I have worked in advertising, forensic and acute mental health services, and currently work as an investigator in a major crime unit for the police. My experiences in these roles have taught me that truth really is stranger than fiction, and have inspired me to write this novel. REALITY FILTER is my first novel — a standalone with series potential.
Please find attached my opening three paragraphs and synopsis of Reality Filter. I hope you enjoy the opening chapters, and I look forward to hearing from you. I would cut all that go for a simple — Thank you for your time and consideration.
My main feedback would be to cut out any words and sentences that don't serve to the plot and use the word count that you've saved for being more specific. For example:
— How does the shadow specifically affect Geoff and Kelly? What kind of deterioration does Kelly suffer?
— How does she exactly defeat the shadow?
— Why would the shadow accept a trade?
— Why can she suddenly see the imps if there's a reality filter?
And so on...
I hope this helps and good luck with the editing!