Pick the right person
Not necessarily the closest. Someone steady, not currently the subject of your difficulties, and able to hear something without acting on it. A friend is often better than a partner for exactly this.
Most people read this alone and that is a legitimate way to do it. Involving one person changes the outcome measurably in one direction and costs something in another, and the trade is worth understanding before you decide.

Not a recommendation. Six differences, stated so you can weigh them against your own situation.
Complete privacy. You can read the uncomfortable chapter without anyone knowing you needed to, which for some readers is the only condition under which they would read it at all.
Someone knows what you are working on, which is exposing and is also the thing that makes it harder to quietly stop.
No pace but your own. You can spend a month on chapter four or read the whole thing in a weekend.
A loose external rhythm. Being asked about it occasionally is a mild and effective form of accountability.
Nobody notices which chapter you keep skipping.
Someone might, and the chapter you avoid is nearly always the relevant one.
Insights stay internal and are easy to lose within a week.
Saying something out loud fixes it. This is the largest single advantage and it is why the effect is not subtle.
Nothing to negotiate. No risk of it becoming their project.
A real risk that it becomes something you are doing for them, or that they start marking your progress.
Stops the moment you stop, with no friction at all.
Slightly harder to abandon, which is either the point or an imposition depending on the week.
The failure mode is not involving someone — it is involving them badly, which turns a book into a supervised programme.
Not necessarily the closest. Someone steady, not currently the subject of your difficulties, and able to hear something without acting on it. A friend is often better than a partner for exactly this.
Specifically, and it is usually much less than they assume. 'Ask me about it every couple of weeks' is a complete brief. Without one, most people default to checking on you.
One sentence about what a chapter named, rather than the chapter itself. Handing over the reading transfers the work and produces a discussion about the book instead of about you.
The written answers are the honest part and they stop being honest the moment they have an audience. This is the one boundary worth being firm about.
'I stopped for three weeks and I've started again.' Nothing more. An account of why turns a gap into a confession, and confessions make the next gap harder to admit.
Including the two versions that most often go wrong.
Cautiously. A partner has a stake in the outcome, which makes it hard for them to hear a chapter as information rather than as an explanation of an argument you had. It works when the relationship is stable and it is a poor choice during a difficult period.
The best version of 'with support'. Tell them you are reading it, bring two dated lines rather than a chapter, and let it feed the session rather than replace it. There is a page on this site on the mechanics.
Reading groups for this material exist and are rare. Where they are facilitated they work well; where they are not they tend to become a place where the most distressed person sets the agenda.
Then read it alone, and it will still do most of what it does. The advantage of company is real and it is not a prerequisite. A written log substitutes for some of it — saying something to a page is not nothing.
Fine, separately, with the understanding that they are reading a book addressed to someone else. What is worth avoiding is a shared copy with shared annotations.
Involving someone, by a clear margin. Whether that outweighs the privacy cost is genuinely personal and this page is not going to decide it for you.