The idioms used are, "put the cart before the horse," and "get back on that horse." Mixed metaphors always make me laugh :-)
There are two other horse idioms I say quite a bit; here's a sample conversation using both of them.
FRANK: I think I am going to ask Susie to marry me.
BOB: Hold your horses, didn't you just start dating last week?
FRANK: Well yes, but you and your wife got engaged the week after you met!
BOB: That's a horse of a different color. We had been dating online for years!
If you haven't figured out what these idioms mean yet, here are some quick definitions:
Put the cart before the horse: To do things in the wrong order (worry about something before you've gotten to that point).
Get back on the horse: Get back to doing something (normally after a failure)
Hold your horses: Slow down!
Horse of a different color: A completely different thing than the one previously discussed. Normally used to