Yes, there are good and bad things, but most of your mental energy should go to perceiving what is the case rather than making the evaluative judgments about what is the case.
Consider the difference between: "This sentence is bad" and "This sentence doesn't quite mean what I want it to mean." Or: "I am a bad cook" vs. "This egg needs to cook for 30 seconds more for it to be perfect for my taste." Even when your ultimate goal is to evaluate something, the negative and positive emotions get in the way more than helping. You don't really have to worry about being too unemotional, because everyone will still keep feeling emotions. It's hard not to.
Try taking some of your habitual judgments and re-casting them as perceptions.
Don't try to "think positively." This is a trap. In the first place, there are negative things, and so thinking positively about them cannot happen. You can't just turn your thoughts around by an act of will. Instead, try to take some things out of the area of judgment altogether. Those will be things you have decided you don't have to care about.
Of course, some people will say that some people are naturally oriented more toward judgment than perception. That might be the case, but even so I think everyone uses both perceptions and judgments to some extent. The point is to make more skillful use of both.
Buddhism has the idea of skillful thought. I don't understand this in its actual Buddhist context, but with that disclaimer I think you can make your thoughts more skillful simply by perceiving whether a particular thought that pops into your head is a helpful one. Once again, it is not a matter of being positive or negative. A thought that is negative in some sense might be helpful.
For example, today I was trying to figure out what keys songs were in by listening to them with my keyboard on my lap and trying to play along. I learned that I lacked this skill, but it was not a particularly negative thought, but a skillful one in this context. By the same token, a positive thought might not be helpful either. You can appreciate its pleasantness but then let it go. For example, you might say: "Oh, I've mastered that skill so I can move on to work on something else."