How to focus on a single area during PhD?

The main point of a PhD is to learn how to be a scientist. Involved in this is to focus on the work that needs to be done but also to pick up the necessary skill to solve the problem. As an advisor, I would get nervous if a student kept moving into new areas without a plan as to what these would be useful for. So from that perspective, if a student made good progress on the research, diversions would not be a concern; without progress, it would be a problem. Where you stand in this is not for anyone to say except based on a discussion between you and your advisor.

During my own PhD, I spent a fair amount learning tools that were only of marginal use in my own work. I am now very happy I did because as now a long-time faculty member, I have come to realize that the time I had as a PhD student to immerse in topics, is hard if not impossible to recreate after the PhD. I therefore advise PhD students to use their time wisely since the tools they learn during their PhD make up the core of their future toolbox. Contacts with other researchers and research directions is a similar issue in my mind. BUT, I always had in my mind that I needed to show progress and stay with my own research tasks as a priority. Balancing between the core work and forays into other areas is a necessity.

From your question, it does not sound as if you have a good balance and I therefore think you need to discuss the thesis work more with your advisor to make sure it is clear and structured to you. Only then will you be able to see your won progress and judge when you are on track.

In terms of ways of keeping focused, I think it's important to keep in mind that it is your job (as in, full time employment) to produce a cohesive block of research. Learning other things is also part of the job, but a smaller part. It might be helpful to allocate specific bits of your time to different tasks you need to do - eg spend some mornings reading new stuff related to your thesis, afternoons on doing the actual research, Friday afternoon reading whatever you're interested in (I'm not saying this is the right balance, just an example).

There's nothing wrong with being curious - that's crucial for being a successful research scientist. Your supervisor telling you that you are overly curious is with respect to completing your PhD studies in a timely fashion.

You have started to discipline yourself and to concentrate on one topic at a time. There is a potential problem of finding interest in a number of topics on first look, and superficially getting involved, then getting attracted to something else before completing something substantial in your previous topic. As Peter notes, the PhD is the formal process of finally demonstrating your capacity to be a scientist. Part of that is dedicating yourself to a topic, addressing it with all the skills expected of a professional scientist, presenting your results and drawing a suitable conclusion. You need to do these things. I think your supervisor is concerned -- rightly -- that you are unlikely to do this if you continue to allow yourself to be distracted.

Strategies - Discuss with your supervisor a topic that you agree is mutually interesting and shows promise for research. Have that topic more in the forefront of your mind as you go about your work. Write it down and stick it to your computer monitor. Ask yourself if what you are doing is more or less likely to further your progress towards achieving what you need to do to be awarded a PhD.