How can Standard deny my claim and expect me to work when I am disabled?

I am 61 and disabled with back, knee, kidney disease Stage 3, hypothyroidism, neuropathy, migraines, IBS, sleep apnea, insomnia, GERD, minimal concentration and focus, depression and PTSD. I have LTD through my employer which I am no longer working and life insurance through them which my employer continues to pay because I am disabled. I received LTD for the 2 years and applied for LTD for medical. They denied me. The life insurance says that I am totally disabled which is why my employer will continue to pay my premiums. If that is the case then how can LTD deny me stating I am not disabled? With all my health problems and medications how can they say I am not disabled? I have worked full time for 41 years so it’s not like I have been a dead beat. At my age all these problems I have makes it hard for me to do anything. How do they expect you to work and think an employer will not fire you when you can’t sleep, you have to run to the restroom all the time, your neuropathy gets worse sitting for long periods of time, standing and walking is impossible other than for short periods of time, your concentration only lasts for about 45 minutes, you fall asleep at work, and you miss so much work because you can’t sleep?