The American had qualified a strong fourth in Qatar, eight tenths of a second down on team-mate Jorge Lorenzo, but faded dramatically as the race wore on.
He eventually finished down in 11th, in stark contrast to Lorenzo, who claimed victory almost 60 seconds up the road.
Spies says his underlying faith in the bike and himself has not been shaken, and that he therefore wants to write off Qatar and start his season afresh in Spain.
"After a tough start at Qatar my goal at Jerez is starting the season again," Spies said. "I'm going to do the best I can to deliver.
"We had a good Qatar weekend except for the race, we know our pace is strong, the bike is competitive and I'm riding well so we should have a successful weekend.
"I'm feeling good, I haven't had great results at Jerez yet but we had a really positive test there so I'm confident."
Spies's stable-mate Lorenzo, who leads the riders' standings as a result of his Qatar win, said he was expecting Honda's Dani Pedrosa to be his main threat in Jerez.
"We were very strong [at Jerez] in our test and I have always had good results," Lorenzo said. "Qatar was a perfect start to our season; we worked very hard for that incredible victory.
"The competition will be tough; I expect Casey and especially Dani to be very fast."