The mistake I'm talking about this week is not being focused.
You have lots of skills! Awesome! Guess what? Everyone else has lots of skills too!
The more ideas you try to cram into your cover letter, the more forgettable you are going to be as an applicant.
When preparing to write a cover letter, I suggest listing out one or two — no more than three — things you want to highlight about yourself and why you're a good candidate for the job.
This might include a skill that you think is vital to succeeding in this job and about which you have an excellent, memorable, very brief story of how you showcased this skill in a previous position.
This might also include something that is not necessarily essential to the job (as in, it's not listed on the job description), but which gives you a leg up over other candidates. For example, the majority of candidates for a data or engineering job are going to be technically skilled in their field, but how many of them also have extensive writing, speaking, or other communication experience?
Rather than trying to show how you can do everything, focus on what you bring to the table that's unique or memorable. Tell a brief story about your most impressive accomplishment that highlights relevant skills.
At the end of the day, a person reviewing a large stack of applications is not going to say, "Let's bring in that person who has a whole bunch of skills and did a whole bunch of stuff." They're going to say, "Let's bring in that person who was able to raise $50,000 in a single month." Or "Let's bring in that person who won the Toastmasters competition. We could use someone with good public speaking skills here."
So try this. Before writing your cover letter, complete this sentence: "When a hiring manager thinks back over their list of applicants, they're going to say, 'Let's bring in the person who _________' and call me for an interview."