A cover letter is a formal business letter, which means it has a specific format and shouldn't include slang or abbreviations the way an e-mail to a friend might. This doesn't mean, though, that it can't be conversational. I've seen too many people fall into the trap of using what I call "cover letter speak": overly flowery language that obscures what you're trying to say and makes your cover letter a chore to read.
One clue that you've fallen into this trap is an abundance of $10 words like "implementation" and "strategize." Words that you rarely use in everyday conversation are unlikely to be the clearest way to communicate your ideas.
As a hiring manager, this kind of writing is frustrating to read because, in the best case scenario, this is an excellent candidate who is hiding the great things they've done in a soup of language, and in the worst case scenario, this candidate hasn't done much of anything and is trying to dress up their non-accomplishments with big words.
It's possibly to write conversationally without being too informal. This is the balance you want to strike in a cover letter. Remember that you are working to impress the hiring manager through your great fit for the position, not for your ability to write in overly complex business language.
If the hiring manager calls over a colleague to say, "Listen to this," while reading your cover letter, it's not going to be because of your vocabulary and sentence structure. It's going to be because they were struck by your enthusiasm or your story about a past accomplishment.
In addition to having someone proofread your materials, consider reading your cover letter out loud to someone. If you find yourself tripping over words, or your friend has to say, "Could you read that again?", you may have gotten caught up in cover letter speak.