Should I finish my PhD?

Your mental health is the most important factor in this equation. PhD's are hard, and demotivating, and exhausting. But the best PhD is a completed PhD! It sounds like you've got everything you need to finish, you just need to full together the threads that make up a thesis.

Often people get most of the way through a PhD and have not taken a holiday in years. When was the last time you took a holiday or just some time off?

Make a plan of exactly what is needed to finish. What are your publication requirements? Structure out what the thesis will look like and define any loose ends that need to be finished. Make a list of these things and plan out - week-by-week when you want these completed by. If the end is in sight, you'll find that motivation will return - like a marathon running who has hit the wall but can see light at the end of a tunnel. These feelings are so natural - almost everyone I know who did a PhD experienced it!

And with regards to you thinking your thesis is mediocre at best. EVERYONE thinks that! I'm not going to tell you that it's better (because I don't know). But statistically, half of PhD graduates wrote a thesis that is average or worse than average. I know mine certainly was! Who cares? If you do enough to pass, then it really doesn't matter. And the saving grace is that very few people will ever read and understand the thesis enough to work that out!

You've got this! Remember to prioritise a good work-life balance as in the long run, it will serve you far more effectively than if you crammed for the next year!

I am very sorry that you are going through this.

It is a very disheartening and difficult time for you. I am not trying to minimize your agony, but I think you should take the time and celebrate how much and how far you have done. You have a few publications, that is brilliant work. You are only a year away from completion and it sounds like you just need to start writing, that is a fantastic platform from which to be sorting out your future. On the basis of your publications alone, you can do many things even if you do not finish your PhD...

Having said that, burnout is quite common and distressing. Unfortunately, burnout is quite poorly understood and there is no accepted "treatment" or strategy. There are plenty of stories of taking time out, doing the yoga/meditation/health diet thing and being able to bounce back. Burnout is serious and many academic journals even Nature has taken time to educate and discuss how serious it is. Self-care is important but taking time out and having a break away from the work is also an important way forward, only something you can decide (Rutgers article).

Your feeling that your work is "worthless" is also very common. It is important that you realise that change will not occur after a 3 to 5 years of intense study, the worth that you give over the rest of your 80 to 90 year life will far outstrip this artificially stressful but short period of time.

Compounding the pressures is the sense, at least according to the economics Ph.D. candidates surveyed by the Harvard researchers, that their work isn’t useful or beneficial to society. Only a quarter of the study’s respondents reported feeling as if their work was useful always or most of the time, compared with 63 percent of the entire working-age population. Only a fifth of the respondents thought that they had opportunities to make a positive impact on their community - Atlantic 2018 article

You may also be clinically depressed. Depression among PhD students and graduate students is common. A 2015 Berkeley study found that nearly 47% of their graduate students were depressed. The highest rate of depression on screening was in arts and humanities fields (64%), higher than in STEM (43-46%), the social sciences (34%) and business (28%).

Unfortunately, few graduate students will seek help and most will likely blame themselves, consistent with impostor syndrome or perfectionism, qualities selected for in graduate school. So yes, it is very important that you seek help, your university would have student support services. It is important that you support and care for yourself in this very tough time. Seek professional and expert help, you have so much more to give after this unfortunate spell...

The other answers already provide you with excellent advice, I'll just add a couple points along the same lines:

• You're not alone: it's very common to feel depressed towards the end of the PhD. Talk to fellow PhD students if you can, you'll be surprised to discover that even among the ones who seem to be doing very well many go through this. This can help you realize that you're not being fair with yourself: your PhD work might not be as perfect as you hoped it would be, that doesn't mean it's bad or even "mediocre".
• Try not to entertain these self-loathing feelings: seek professional help, take a holiday break, contact some old friends or spend some time with your family... Whatever way works for you to change your mind, go for it.

I really feel that the reason I got into this situation is because people that were supposed to help me think very low of me... And they just left me to deal with my problems all by myself. Probably because they think I wasn't good enough to deserve much of their attention.

I don't know your supervisor(s) but I can confidently say that your interpretation about why they didn't help you as much as you expected is completely wrong. There are plenty of reasons why some supervisors turn out to be moderately helpful, the most common ones being: they are too busy, are not as knowledgeable as you imagine in your topic, started other lines of research and lost interest in yours... or just have a very loose approach to supervision. Supervisors are not perfect either, it doesn't mean that they judge you badly. This interpretation is the result of your low self-esteem.