But perhaps believing the truth is against my interest. If I believe that my work is important to outcomes, I will work harder. If I believe my work has little importance, I may become lazy or seek side-hustles. Should I convince myself that my publication count and thesis quality are more important than is true?

So I think the answer is somewhat complicated, and requires unpacking a few things. The big thing is that it is possible to commit without belief, many people find this hard to do without expending willpower, and so find it instrumentally useful to lie to themselves about what they believe, only possibly to find out their beliefs are not very malleable and they can't successfully lie to themselves to achieve this end.

The best situation would be to believe the truth and do it anyway. This requires a level of non-identifcation with the belief, though, such that you can successfully invest in an uncertain outcome and be happy with the expected returns rather than the actual returns.

If that's not possible, next best would be setting up incentives such that you don't have to change your belief and can maintain beliefs you believe to be true but are nonetheless incentivized to do what you want yourself to do. This is painful for a lot of people because they feel themselves fighting the incentives they themselves set up, but it's an option.

Epistemically the worst option is to lie to yourself, but also probably the least painful if the first option is not available. It will work so long as you can maintain the lie, but you might not like having to do all the work to maintain it, and it'll inevitably poison other beliefs by the need to maintain the network of dependent beliefs that prop up the falsehood you're maintaining. Not recommended, and you'll create a lot of harm for yourself to unravel later.

Limiting my comments to the question at hand, but if you asked for my general opinion, it would be to have alternatives that let you out of the frame of the situation you've created so you don't have to do this.

Some thoughts:

• Regardless of what the truth is, it seems like working hard is the best way forward, rather than getting lazy or seeking side-hustles.
• Is it possible for you to willingly choose to believe something if you don't actually think it's true? To what extent? I don't think it's a yes or no question.
• Related to that, are there any downsides of anxiety? Ie. if you go down the path of believing something that isn't true (or is stretching what is true), will you have a nagging feeling in the back of your mind that produces anxiety?
• What are the downstream impacts of deviating from truth? Imagine the following scenario: reality is that your work doesn't matter much; you choose to believe that it is for instrumental reasons; down the line there is a ton of evidence that your work matters very little but since you took the blue pill you aren't able to process this evidence and update your beliefs. It seems to me that in order to justify deviating from truth, you have to be pretty confident that it's the right choice because you give up some future flexibility.
• I'm glad you're asking this. I this idea of instrumental truth often times gets dismissed too quickly and I think it's something that is worth talking about.

I'm sorry if I'm missing a point, but here are two cents on the subject : 

"Should I convince myself that my publication count and thesis quality are more important than is true?"

I think it's hard to break down the problem at this only question : maybe the answer to "success" is to work differently, and not harder, thus it may be dangerous to be entangled in the sole idea of "working more". Various skills are necessary and focusing too much on something may be harmfull. 

(Also I'm only pointing this because my global opinion has already been expressed (: )

I can tell you this is a thing in PUA community.  Believing that every girl wants you is incorrect, but more helpful than (also probably incorrect) belief that no girl wants you.

The real solution is to drop out unless you are clearly a superstar or you are extremely close to finishing your PHD. No need to deny the truth. 

I say this as someone who wasted three years in a PHD program. Luckily I got out when I did before I wasted more years or started a postdoc. If you are not a superstar start planning your exit.

If you have goals that justify working hard, like the satisfaction of a job well done; or hopes for a professor position; then do  that. 

If your goals justify not working hard, like enjoying your doctoral stipend while gliding through and  then moving to some other profession, then do that.

If the only reason you are focusing on your thesis is to get a professorship, it sounds like you are not into doing the research for the right reasons.

How about choosing research topics that seem important enough that even in the case you don't get professorship you could say it was worth investing yourself into them?