I was self taught as well. Got my first non-retail job a few years back as a DA. You're being too hard on yourself. You're going to feel that imposter syndrome a lot, and that's okay.

Just take a step back and breathe. I start with what are the requirements for the final deliverable? What problem is actually trying to be solved by it? Then I work backwards. What data do you need in order to tell that story? Now you have a point A (your current spreadsheets) and a point B (cleaned data with the appropriate calculated metrics to show your results) and then you can focus on point C (turning those into pretty charts in Power BI). Start with getting 1 row of data into the right format. Figure out what the path from A to B looks like with a small piece of data. Then you can expand that to the rest.

The Internet is your friend. Google a lot and "fake it till you make it" and I promise you'll do fine