The two may be related. I think it's hard to convincingly bullshit yourself - your brain knows what you're doing. But if you successfully bullshit other people, some of them will start to reflect your bullshit back at you - providing external validation for it. At some point your brain may forget that the only reason people around you believe something is because you tricked them into it, and you start truly believing it yourself.
This is used in various belief systems to instill and reinforce deeply held beliefs in the absence of any evidence apart from the manufactured social proof. If one is experiencing doubts, they are told to try telling others that what they doubt is actually true. One example of the phrasing that might be used: "You have to share your testimony to strengthen it."
No doubt the technique of manufacturing consent through reflected BS is pervasive elsewhere too, in marketing, politics, etc.
> I think it's hard to convincingly bullshit yourself
That is entirely dependent upon the personality and goal of a given person.
For some people the brain knows what it wants to know, such as selective data necessary for group conformance. For some other people the brain knows only what it sees and hears. For some different people will doubt what they see and hear until after several iterations as to meld conflicting reports.
There is a common expression for people who commonly and easily lie to themselves: Talking a big game. In that case everybody knows its bullshit except for the person talking.
An argument of Elephant in the Brain (https://www.elephantinthebrain.com/) is that a key component of bullshitting others (paraphrasing, somewhat) is that you must also bullshit yourself, in order to be convincing.
Not really, you believe it yourself. You don't put up much resistance, it's just kind of "this is convenient, therefore it must be the truth, and these pesky contradicting facts probably have some other good explanation".
If you believe it yourself. What I'm saying is, I don't think you can think yourself into believing something on purpose, when you didn't believe it before, and know that thing is wrong. You'll need a round-trip through other people (external validation), for your brain to stop seeing "motivated reasoning" tag on arguments.
