Knitting a blanket on a circular needle

I’m about 12 rows into this blanket of 176 sts and I just realized the blanket joined. Can I fix this without starting over?

You can cut the yarn where it’s joined, but you won’t have long enough tails to weave them in. You’ll probably have to rip out to the join, which isn’t that difficult and less time than it takes to rip and start over. There are videos on the Tips page under Fixing mistakes that show you how to put your needle in. Rip back to where you joined them, then insert your needle into the sts as you pull out the sts on that last row. If you do it one by one, inserting the needle as you pull on the yarn, it’s easy to do, and you won’t drop any sts as long as you don’t stretch out the piece.

When I knit across, it looks like I’m joing the two sides. How do I keep from making the same mistake again?

It sounds like your trying to knit in the round which would join the two ends. When you knit flat you just flip the project so you are knitting on the other side, just the same as if you were using 2 regular needles. Just pretend that the cable between the 2 needles has been cut.