Sounds just like slightly bad timing and stroke. I have had this myself, keeping the rod tip slightly higher on the forward shoot helped, and feathering the line and bringing the rod down parallel to the water at the last second, the dip in the rod and feathering is something you will have to try.
Like you, untaught. The biggest problem is you only know something is wrong when you get a big problem. An instructor would correct this in a minute, but it is interesting and satisfying to plough your own furrow, so to speak.
Also, check your line to braided loop connection is fluid. 2" of line/ braid set solid with superglue, or a soft spot ( large gap between end of line and loop ) will have equally bum consequences, especially if your ( like mine ) technique is a bit ropey.
Enjoy learning and asking questions!!