I'm sorry to hear that your kitty has megacolon.
I've read A LOT about it since it was believed that my kitty had it (until yesterday). It is something that requires long-term medication, but that's all that some kitties need to manage megacolon. There is an operation that's usually left as a "last resort" type of deal, but it appears to have a high success rate from what I've read.
I've been dealing with constipation issues with Bailey for over a year, so I know what you're going through. She's been getting lactulose ever since her first bout of constipation, and we've had to increase the dose quite a few times. She's only needed one enema since we got the dose right, but she's been having minor constipation more frequently lately. The x-ray she got yesterday wasn't indicative of megacolon, so I'm really not sure why she gets constipated.
Anyway, I've found it to be a HUGE pain for both of us to administer the lactulose manually with a syringe or a medicine dropper (she hates it regardless). I was giving it to her in her canned food, which was really nice, but then I'd have to give some to her manually if she didn't eat all of her food.
Cats generally are not fond of the lactulose since it tastes sweet and is super-sticky. I end up having to wipe Bailey down every few days with a wet towel and comb the lactulose out of her fur since she gets it EVERYWHERE.
I know this is easier said than done, but try not to be too hard on yourself about not noticing it sooner.
I still feel like a jerk sometimes about how SEVERELY constipated Bailey was by the time I rushed her to the e-vet the first time. She puked a few times over a couple of days and wasn't eating much. She was also being pretty antisocial. I still didn't even suspect anything was wrong until she started straining to poop in random places throughout the house (she managed to drop a couple in the bathroom and living room). Then I went to pet her and when her tail went up, I noticed her anus was protruding about a 1/2 inch. I was horrified!! Looking back on it, I feel bad that I waited to do anything, but constipation had NEVER been a concern with the cats I had growing up. It was something I never even thought to look out for until it happened. I think it's one of those things that you just don't really think to look for on a regular basis. Now I'm constantly lifting her tail up to make sure her bum looks normal (well, I don't think it'll ever look completely normal, but normal for her). She must think I'm crazy.