hmm... perhaps you could try a new tactic? Spice things up a bit - do exercise that is fun (sport? dance? yoga?) and perhaps lean more on diet for weight loss so that you will see results even if you don't kill yourself at the gym
I have found lately that it is much easier to be motivated and stick to something if I run to some sort of program or plan that I set out rather than generally living a healthier lifestyle (if that makes sense?? like, I know what "good" and "bad" food is but just sticking to good foods and avoiding the bad doest really entertain me so much...) What I am having fun doing at the moment is working to a program of combined calorie counting and different menu plans - I joined a couple of different sites for meal ideas and enter my foods into myfitnesspal so I plan my days ahead and do shopping to fit in with the recipes I want to make etc - because it is all preplanned it is really easy to stick to my calorie goals and I am finding this is resulting in weight loss so far, thus motivating me to keep going.
As an alternative, I have been really enjoying reading the philosophy of living in the "French Women Dont Get Fat" book - if you go to the website for this she details the key points of how to eat and live well so you get the idea without actually buying the book - but an entertaining read if you are in the mood
I am not saying specifically do these things - I am just saying what is working for me and what I find interesting right now...
I totally get what you mean with the motivation moving between those few kilos that you are always seeing - I kept dropping between 82 and 76 depending on whether I was being good or not that month but because I had been there before getting to 78 was nice, but not enough to feel like I had really done much... but now that I am running to a specific plan that I feel I can maintain, I am moving into territory that I havnt seen in years, every 200gs lost is exciting to me!! and I dont see any reason to stop while this is working, or at least not for any length of time (I still eat out or have a rediculous dessert with a friend or crazy night on alcohol etc - not good physically but mentally necessary imo
You could buddy up with someone on here and compare weekly weigh ins or join the current 12wk challenge