I have never personally owned a bit that must be replaced every few months - what's the bit made of and what goes wrong that it must be replaced? Is it because the bit gets rusty that you replace it? If so, then you don't understand what "Sweet Iron" is. Sweet iron is a metal that's intended to rust soon after you begin using it. The rusting is thought to promote salivation which creates a sensitive mouth.
Myler bits are made of the highest quality materials in my opinion. I'm sure there are others, but I've never found one yet that I like the quality of workmanship, the technology in the mouthpiece to avoid pain infliction or the process of communicating bit level and education that Myler has.
I ride my two youngsters in a Myler comfort snaffle - it's a d-ring snaffle with a stainless steel mouthpiece that has copper inlays and a copper roller in it. When I switched from a traditional two-piece mouthpiece snaffle, within hours the difference was as night to day. My bits get crappy with slobber, dust and grass goobers - a simply warm water wash with some mild dish soap has them looking like new. I've ridden my 8 year old for about 3-4 years in the same bit - it looks like new.
These bits aren't cheap, but I swear they'll last longer than about any other bit out there. They have interchangeable mouthpieces and cheekpieces - and are available in french link-type mouthpieces with any kind of cheekpiece.
I started my younger horse on a comfort snaffle with a twist made of Sweet Iron. This last summer I noticed that she's really playing with the bit. I switched her to the other stainless/copper smooth bit I use on the other horse and she quieted right down. The sweet iron was promoting her playing or there was something about it she didn't like.
I suggest you visit www.mylerbitsusa.com - visit your local retailer, or get to an expo where the Myler brothers are speaking. there is a book and video you can get for $10-$15 from about any retailer - the video helps you understand bit resistance, how Mylers rating system of bits works and why each bit is a certain type and give you some ideas of how to 'feel' what the horse feels when a certain bit is placed in their mouth and engaged. I feel that if you watch the video and understand how the bits work, you will learn a lot and be able to pick up about any bit and get an idea of how it works and feels to the horse.
After you watch the video, you can assess then whether your horse is a focused on you as it should be or whether it may be un-focused because of the bit type, then start either asking questions or trying bits to figure out what works better. Myler (Toklat is the distributor) has a rental program, but the tack shop has to invest in the bits to actually rent. If you can watch the video, then rent a different bit to try, you can be better assured it will work if you buy it. I suggest taking your current bit to the retailer, telling them what's working, what's not and discussing your use of the bit they should be able to get you into something that will last you years and years with success.
I forgot a couple points about the rubber bits - if you find the exact bit you're using with a rubber coating, you'll be lucky. You must consider the thickness of the mouthpiece if you do, because a thicker mouthpiece may be less comfortable to the horse. I spoke with a gal this weekend who found a horse who chomps at any bit you give him, he likes the rubber ones better, but tends to destroy them. She wraps her bit in some sort of bit wrap I'm unfamiliar with.
Regarding bit resistance - you must determine whether the horse is demonstrating it or not. If the horse carries that bit quietly and comfortably in his mouth and you have his full attention, he's fine with the bit - but if he slobbers, lips, bobs his head, sticks his nose out - anything other than quietly holding it awaiting your cue, he's either uncomfortable with the bit or the way it's being used. Don't give the horse human attributes - just because you think it must be yucky doesn't mean your pony does. Allow him to tell you what he thinks.
As I type this I'm pretty convinced that you're seeing the Sweet Iron rust as it should - which usually takes a month or so of use, then you're thinking it's junk because it's rusting - it's not, that's what sweet iron does.
But do know why the french link is a different bit than a two-piece snaffle before going to look for a different bit. You can totally go backwards in your communication iwth the horse if you change bits without knowing why or how the horse perceives it.
good luck!