This might sound dumb, especially since some prevailing advice is that sometimes you have to 'kill your babies,' but I think you just need to fall in love with your comic. (Again, if you haven't already.) This is going to be a novel, so stick with me here!
Early on in my comic's journey, maybe four months after launch, I had to reexamine it closely to figure out what about it was not working for me. That resulted in me hammering out the script, actually renaming it (only recommend in special cases), and making sure it was really the story I wanted to tell rather than going for the cheap shot or easy way out. The more I work on my script (it is done but I am always tweaking it) the more I love it. And I love this story so much that I know what needs to be done, even if that means 'killing my babies.' By that I don't mean characters, I mean the entire story and its ideas -- don't be so attached to it that you can't change it. Entire chapters have just been tossed out or set aside, plot points removed or changed, etc. Knowing your comic well helps you understand its strengths, appeal, which can help you target your advertising.
So, staying motivated:
One: Get to know your comic again. What is it about your comic that is awesome? If you don't love it, no one else will! You certainly won't want to spend endless hours making the darn thing!
Two: Try to find one thing to do when you feel despair. For me, it's pages. Forcing myself to work on pages always focuses me and balances my mood out.
Three: Network with other artists, which is what you are doing here! You know we all have these problems to some degree, and everyone here has posted their methods of staying motivated.
Four: Be tough on yourself and be realistic about your time, and this is the primary reason I force myself to do pages when I feel despair. When you are doing a comic, the pages are the most important thing.
Everything else is for vanity -- illustrations, etc. When I studied fashion design I was told that the shows were for vanity; all your hard work behind the scenes and the every-day designs you make to sell are the reality, and what you should focus more on. I keep that in mind for comics. When I tripled my weekly updates, the amount of illustrations I did decreased sharply. Sketches are good though, they help you warm up, learn new things and reinforce good habits.
Five: Be active in advertising, networking, and talking with readers. Communication is very important for comics, especially young comics. Remember to keep an allotted amount of time for this though, because you don't want these activities to take over your content-creating time.
Six: Recognise your working style so you can head off burn-out. If you gotta take a break now and again in order to avoid a longer, more painful burn-out, do it.
Maybe make a motivational music list as well? Honour your moods. Sometimes I listen to four songs on repeat, but for whatever reason, it is what I needed for those pages.