Long term antibiotic use is highly undesirable because as you have noted, the bacteria eventually become immune. Each time you take an antibiotic, the few bacteria that survive are the ones that have developed immunity and then reproduce, which are then the ones that become a new infection.
There may also be more than one kind of bacteria that are present, not just e.coli. While the most common, it's still only about 85% of cases. You can certainly have more than one.
Now there IS a difference between having enough bacteria colonizing to be classified as an infection by a test lab, and just having some bacteria. So if the cut-off for an infection is say 1000 bacteria, and you have 800 bacteria, the lab is going to say you don't have an infection from your sample, but it does NOT mean you don't have bacteria. And over time, those few that are left are going to be the ones that are immune to Macrobid.
There are a couple of things for you to think about. First, determining the cause of the recurrent infections. It's usually one of three things: hygiene, insertion, or physical abnormality (in that order). Keeping new bacteria out is the most important thing towards preventing another infection.
There are other things you can do too, especially with recurrent e.coli infections. You can try a cranberry product on a daily basis, such as TheraCran (if you don't like cranberry juice). While cranberry doesn't cure an existing infection, what the proanthocyanidins in cranberry do are bind to the p-fimbria of the e.coli bacteria so they can't adhere to and colonize the bladder wall. Note that the proanthocyanidins bind to your blood too, so it's important to consume your cranberry product twice a day since its activity diminishes over time. You need to take it about every 12 hours. This kind of measure is popular with people who have recurrent UTIs because it's not a drug and specifically deals with e.Coli if that's know to be the culprit. And it doesn't develop into a resistance either.
You should also probably be taking a good probiotic. Your body's own immune system is your best defense against acquiring another infection. Keeping yours healthy by restoring it with no less than a month of probiotics is a good idea, ESPECIALLY since you have been taking antibiotics so long. Make sure you take them at least 2 hours after an antibiotic dose or you're just chasing the good army with its death (antibiotic). The one probably best to take while you're still on an antibiotic is sacchromices boullardi (sp?), but after you stop your antibiotic, you can take any more comprehensive probiotic. It will help your digestive system too.
So back to hygiene, the number one cause of infections...hopefully your urologist has explained in detail all the things you need to do like always wipe from front to back, not the other direction. There's a British woman named Angela Kilmartin who has very practical advice on her web site and book about dealing with UTIs and cystitis. You may find her approach interesting.
Good luck.