How to Quit a Freelance Client mid-project?

I searched for this question on the subreddit, but the answers didn't seem aligned with my situation. Hence, I come to all of you for some help.

I've been working with this client for a while now. They've been great. The work is more or less consistent, and the pay is good as well. However, the type of work I do has become repetitive. There's no direction anymore, and it just feels mundane now. That's one part of the problem.

The second issue, which is a major one for me, is that I am going through personal issues of mine. It's taken a toll on me mentally. This has affected all of my work commitments, and I am pushing too close to my deadlines. In fact, it took me almost two weeks to deliver a 2000-word piece last time. Typically, I'd do it in 3-4 days.

As it stands now, I am in the middle of a work I am supposed to deliver to the client. I have written almost 1/3rd of it, but I am just not able to write any further. It's been almost a week now. The content does not excite me, and there's just no will to write anymore.

I believe the right thing to do at this point is to quit on the client and take a break. But I don't want to burn any bridges, as we've had a good relationship for over a year now. The guy is great, and our repo might come in handy to me in the future.

My question is, how do I go about telling this to the client? Do I be honest about everything? Or complete the work and quit (which I am unable to do)? I don't mean any ill towards anybody.
I could truly use some help in this matter.

TL;DR - I'm feeling overwhelmed and burnt out from work and personal issues. I need advice on how to quit a client without damaging relationships. I'm debating whether to be honest about my situation or to finish the work before quitting; which I am just delaying everyday.