Hi gordon,
Among other things, the LSAT is definitely a test of mental stamina. To do your best, you need to be able to focus 100% from the first question to the last. I often compare studying for the LSAT to training for a marathon. You should be gradually building up your mental stamina in the weeks leading up to your test. Just like you would gradually build up to running a marathon, you gradually work up to being able to focus on all four sections. Start with one section, then move to two, then three, and finally all four. I know of some students who even try doing five sections (once they feel good on doing four) to push themselves, so that four sections may seem easier by comparison. (Of course, back in my day, the test was five sections followed by the writing sample, so you all actually have it pretty easy!)
It is really important that you get sufficient sleep and that you stay healthy. You also should try to study (and take your practice tests and later the real test) at the time of day that works best for you. If you're a morning person, try to study in the morning if possible rather than at night when you may be mentally/physically exhausted.
Other practices like meditation can also help you focus. Meditation can also help reduce test anxiety.
Here is a link to list of resources relating to test mentality (including mental stamina/focus):
https://blog.powerscore.com/lsat/the-ul ... ource-list
Losing focus may also be a product of becoming burnt out due to overstudying, especially if you have a lot of other work/life responsibilities going on. Here's a link to a good blog post on burn out, and why taking study breaks is important.
https://blog.powerscore.com/lsat/bid-24 ... your-score
On a broader note, there's been a lot of discussion recently on how smart phones and social media may be seriously reducing the attention spans of younger generations. The book "The Anxious Generation" by Jonathan Haidt discusses how social media has negatively impacted the mental health of adolescents in many ways, including attention span. The book "Deep Work" by Cal Newport is a good resource on how to improve your focus on any work/studying that you do and also recommends limiting/avoiding social media.