Yeah, that's a pretty common mistake when starting out. Broad match without a solid negative keyword list can burn through a budget surprisingly fast, especially with generic terms like "phone case."
The first thing that made a difference for me was regularly checking the search terms report and adding negatives every few days. I also moved away from broad match for most keywords and focused on more specific terms with clearer buying intent.
Another thing worth looking at is which products actually convert. Sometimes a small group of products generates most of the sales, and it's better to build campaigns around those instead of advertising the whole store equally.
At least you got some sales early on, which suggests the offer itself can work. The issue sounds more like traffic quality and campaign structure than a product problem.