I've found myself in a rut feeling the way you describe several times in the past two years. The first thing I do is try to evaluate what might really be happening to me. Sometimes my lack of enthusiasm was because I as simply working too hard. I also have found that doing the same routine more than a few weeks can also lead me to burn out. I'm not a big fan of repetition.
My interest in heading into the gym can also falter when "life" gets in the way. My goal this year was to strike a balance between spending time with my 4 year old on the days that he was not in preschool. That meant that there were two days of the week that I was not going in. Then there were appointments to keep on the days that he was in school so my gym days were cut there when necessary. Add in the once a month classroom volunteering, PTO stuff, and the lunch group I belong to and things got a little hairy for me this year.
I struggled to get in two or three days some weeks. ~sigh~ Without the reinforcement of those endorphins pushing me in for my next workout it became easier to resist heading in on the days I had available.
So, when this happened I kept in mind how wonderful I felt during my cardio sessions and how relaxed and just slap stupid happy I felt two hours later. I'm not willing to give those up for anything. That's when I know I have to head back in and plow through until I'm feeling great again.
The other thing I do when I feel a rut coming on is research new exercises/approaches/techniques online and in the library. I may take a month and research functional fitness or maybe challenge myself with all bodyweight exercises, currently I'm thinking something like yoga might be a nice addition as well. One of my favorite things is to watch CrossFit videos for ideas and inspiration.
You may also want to invest a little money and hire a personal trainer. Just having another person to bounce things off of who is there in the trenches with you is helpful too. Just ask JG.
If all else fails then I get out the photo album from five years ago. Have my little moment and then promise myself to go back in the next day.
I really think the key for me though is consistency in going into the gym and depositing those positive moments and experiencing in my memory as they occur so that I can draw upon them when I'm feeling the way you describe.
Bottom line is we all feel this way from time to time
It does not need to result in a permanent break in all the good habits you are establishing. Sometimes a break really is necessary. It is a time for evaluation and repair. In the New Rules for Women the authors also point this out and plan a week of rest just for this reason.
Good luck and happy researching.