I've been in your position and am currently on my way out of my PhD program by mastering out.
When I first felt that my Chemistry PhD program was not the right path for me I was three months into my first year. I was doing rotations at the time, and everything felt wrong. I forced myself to do homework, forced myself to do research, forced myself to read papers. I had never had to force myself to do anything during undergrad, so this feeling was terrifying. I didn't enjoy any of the work I did and I was starting to realize that I didn't even like doing basic research. I didn't want to be cooped up in a laser lab for the rest of my life. I realized that I wanted out. But it's not that easy.
When you first start having the inkling that the academic life may not be for you, it can be frightening. It can feel like you are a "quitter" or you may have some reservations about what other individuals in your cohort may think or you may feel that you are harming your future career. All of these feelings are mind games that we play with ourselves and they are games that are hard to let go. I just want you to know that all of these thoughts are pointless, and quite frankly, untrue.
Another issue that came along is the guilt of wanting to leave. Since PhD students are getting monetary support in some form or another it can feel very dishonest to our advisors to continue to take support with the intention of leaving. We want to leave gracefully on good terms, but it always seems as though we're leaving our advisors in a lurch. The guilt of this dishonesty began to affect my life physically and mentally. I began having constant anxiety and severe headaches that ultimately landed me in a case of depression. I sought help from my university's mental health program and I'm back on track towards being my old self again.
The point of me sharing all of this is that this is your life. You do not have to do anything you don't want to do. When I first told someone I care about what I was feeling he said, "If I was doing something I didn't like, I wouldn't do it." A PhD is a commitment of time, body, and mind. If you are not doing what you want to do or gaining something in your future career by being in this program, then it's time to move on.
First, seek psychiatric help. Immediately. Until you start to stabilize your mental health, you will not be able to make any rational or helpful decisions for your future. I initially made the mistake of thinking that my issues were not important enough to seek help (compared to other people) and that the counselors would think I was just being pathetic. This is not true. That's the anxiety talking. Get in there and get the help you need.
Second, think about why you initially wanted to go to graduate school. Was it because you wanted to be an academic? Or because you didn't know what your next step should be, so more school sounded right? Are you passionate about your subject? Or is it something that you and others believed you were good at so you should keep on doing it? This was my number one big mistake about grad school. Make sure you are there for the right reasons.
Third, think about your personality in grad school. Oftentimes (especially in the sciences), PhD students are alone. We are in environments that foster individuality, competition, and (to be honest) very little constructive advice. It can be very alienating. Does this environment fit your personality? Personality isn't always a great factor, because you can overcome this issue if you really love your work. For me, it was just another affirmation that the program was not a good fit.
Fourth, try your best to not worry what others (including your advisor) will think of you. Try to control that guilt, that "I'm a quitter" feeling, that embarassment. You are making a decision for you. In my experience, when I told others in my cohort why I was leaving and what my next step was no one was condescending, snide or hurtful. Everyone was encouraging. As for advisors, they know. They can tell in your work. I finally let mine know, and it's been fine. As long as you're doing the work that you're getting paid for, there aren't any bad feelings. No one wants to force you to do something you don't want to do.
Lastly, start outlining what you want your future to be. I started by talking to the graduate advisor and switching to the Master's degree track and finishing the requirements. (This is very important. If you can, finish the Master's degree. You should earn something for the time that you spent in your program and this shows your future employers that when you set out to do something, you finish it in one form or another.)
I then began making a list of tasks and aspects of my previous work that I enjoyed and incorporated that into something I am passionate about. I have been volunteering at charities and non-profits to get experience. I started networking through employment fairs, county fairs, and anything else I can get my hands on. Now, I'm applying to internships and I'll have my Master's degree by December. I no longer feel remorse or am upset by my choice. I'm excited for what is coming next.
The long and short of all of this is: tell people you care about what is going on, get the help that you need, and keep your chin up. Hang in there.