The first million times I tried doing the gym thing, I'd be good for a week or so and then I wouldn't. I think a lot of it was that I would just wander from machine to machine not really knowing exactly what I should be doing. It didn't help that those machines were boring too. But I also wasn't motivated to do it on my own at home, at least not for long.
I finally made a commitment to try again. I decided that it wouldn't do any good to try doing the same thing that failed before so I looked into a personal trainer, and for me that has been a huge difference. I needed that accountability factor - knowing someone is going to be reviewing what I've done (or haven't done) to progress. And based on achieving results (32 pounds in 11 weeks), I have to admit that this was the missing ingredient for me.
When I was checking out different places, I made sure that any gym I was considering wasn't just machines. I wanted variety and now I don't spend a lot of time on machines. I have 1 day a week working with my trainer. She has me do a lot of things and very few involve machines. The other days of the week, I fill with the Les Mills group exercise classes offered. After trying one, I found that having other people around who were going through the same thing with me was encouraging, plus I never do the same class back to back so I never get bored

BodyPump and BodyCombat are my favorites.
You mentioned considering Zumba...give it a try in a group setting maybe to make it feel more like a dance party (a really sweaty one!) than a workout. If you have a local YMCA, they might offer it on a day-pass basis so you can try it without committing to a membership you might not use or spending money on a workout DVD that you might not like.
I wish you luck in finding "the thing" that you actually enjoy despite the effort of working out
