RE: cant go past half throttle
 

How do you know it is not the cat converter kaylyn? P0421 is not a dummy code but one telling you that cat efficiency is below acceptable limit. Given your bogging down and the code, I would bevery suspect of a clogged front warm-up cat. This is located just after the exhaust manifold. Very easy to remove and check. 5 bolts on the top once the manifold heat shield is removed. Remove the short 'a' pipe just after the cat with 5 more bolts and you can get it out to inpect. You could also just see if the engine revs up normally with Cat removed or at least inlet loose to allow exhaust gases to escape. You should be able to see thru the monolith element by shining a bright light thru the oposite side. See link below to some pics of a warm up cat that was causing a huge power loss problem and then suddenly went away when the converter monolith element completely broken down. P0421 was also stored along with a random missfire P0300.

https://www.mazdaforum.com/m_28262/tm.htm