I understand. It's easier to plunge yourself into anything when results are as expected, but when technical issues arise in conjunction with not being able to keep yourself invested enough on just one thing until completion, it can be a very uncomfortable situation.
Let me start by saying that I do a million things at a time, constantly, both for work and pleasure, but I found my own way to never burn out and never lose my motivation. It took me a long time (I'm 43) and quite a bit of pain (life can hit so hard at the worst moments and those moments can last forever), but it works for me, because I really took the time and energy to try things out, while also learning to get out of guilt trips similar to the ones you mention.
Software not cooperating is not an excuse at all, it's really something that can bog you down. I bet you already asked yourself this question, but did you ever consider changing your software, or going for a more powerful hardware? If that can't be done (too expensive or anything else impossible to overcome at the moment), what about trying traditional drawing, even coloring, and then just doing the lettering and/or retouching digitally?
Aside from that, 3 to 4 pages per day is really a lot. I read you're a freelance artist, so I understand you need to produce art for a living, but that still seems like an awful amount of work, or perhaps it's too much for you at the very moment, so I can see why you'd just burn out and feel like doing something else. I think the point is to avoid getting into guilt trips and remembering why you are doing this (beside the money, of course): likely, you draw comics because it fulfills you, it makes you happy, and it makes your heart sing with joy. I believe that if you keep that in mind, you should try to stick to that empowering feeling and work out a sustainable schedule so that you don't burn out and complete your projects without getting scattered all over the place.
So, long story short, if I may make a suggestion, I would work out the software/hardware issues first. With those out of the way, I would finalize a decent schedule for a single comic or two for some time, like a month, and stick to it no matter what. At the end of the month, I would make the necessary adjustments. I did something similar in the past (exactly around your age), and it worked for me.
Just my 2 cents, good luck