How to tell if you have gone too far after you went through and added a bit of zing from poetry to your manuscript. There are no really good answers to that question. It is, in large part, a matter of you own personal taste. My own rule of thumb is to cut back when even I am annoyed at how gorgeous all of my sentences present themselves.
But if you want a couple of guidelines, I guess this is it. Use alliteration sparingly. Top not political speech writers, use alliteration only to mark the most special of passages. Think of Peggy Noonan's insertion in President Reagan's address addressing the loss of The Challenger crew. (May they rest in peace.) "Slipped the surly bonds of Earth" to "Touch the face of God". This is a quote from a John Magee poem, used to full effect to a nation in mourning. This quote comes very close to end of the speech, which had to be rushed, because the State of Union speech could not go on as the nation mourned. The passage, slipped the surly bonds of earth, has both the rhythmic cadence of poetry and combines some mild alliteration. It is not an accident that Ms. Noonan picked up the quote at the alliteration, and it is not an accident that she cut from the beginning of the poem to the end. She has a fantastic ear. (Which is why she can get paid to write these kinds of speeches and I can't.) But it is her restraint, in only picking these small parts, and deploying them in the most effective place that really distinguishes her from other speechwriters. You can do that too. It is a matter of pulling back.
Three questions to ask if you should kill your darling?
One, what purpose does the sentence serve? If it is only there to be pretty, kill it. Be pretty in a sentence that matters.
Two, have I said this before? If yes, you must go back to the other sentence and keep the most effective one. That may not be the prettiest one, but the one that holds the most tension and advances the plot the best. Kill the weaker sentence.
Three, have I been showing off by using this particular device? If the answer is yes, keep one in three. Kill the other two.
I know. It sounds harsh,. That's because it IS harsh. Being blind to one's own faults is a normal thing. This is why we get critique partners and editors. But, learning how to be reflective on your own work is a good thing. The only way to get better, is to practice. It hurts, but then so did your parent's ears when you were learning how to play your instruments. Or you muscles hurt when you decided sitting on the couch was only making you...depressed. You belly aches when you start that New Year's diet because you ate all of that holiday fudge laying around the office. It is NORMAL for it to hurt. But it's in the willingness to not let that stop you that you make great improvements in your writing. Keep trying. Keep getting better. Keep at it. You can do it. Bit by bit you will get better. Never stop learning. Never stop trying. Never stop striving to be the best you can be.
PS: This post was originally meant to run on January 28, 2019. The 33rd anniversary of the disaster. But I just can't. Up until that moment, I had secretly harbored hopes of becoming an astronaut. But my teacher was away that week and we had a sub. That was one of the scariest days I ever spent in school. So I was silent yesterday, and I put this out today, January 29th, 2019 in memory of that crew and in honor of my 4th grade teacher. The references to the speech are for historical and writing purposes and have no bearing on my political orientation.
As for my hopes of being an astronaut, Challenger may have put great fear in me, but my own need for prescription glasses a few weeks later is what really put the final nail in that coffin. (I had not yet learned of my hip deformity, which would have precluded me in every eventuality.)