Coping with the potential loss of a partner in an intergenerational relationship

Someone on Reddit posted a question about how one deals with (copes with) the potential loss of their partner in an intergenerational relationship. I didn't have a straight-up answer for this, but this is was what I responded with: 
I'm from India and I'll soon be 40. I have been in three relationships so far. 
The first one when I was 27 years old. It was a 3-year one with a then 67-year-old American from NYC. I knew that he had chronic illnesses but we were madly in love. He flew into live with me. He had serious complications of his long-term illnesses and had to be hospitalized in India. I then had to send him back home (I couldn't go myself because of visa issues). In NYC, he was admitted in multiple hospitals and eventually was put on life support for a few days. He eventually recovered, but couldn't make it back. We continued for a couple of more years and eventually sought other partners. Losing him multiple times was the hardest part of my life. I was diagnosed with PTSD and related depression and have been on therapy ever since.
The second relationship was with a 73-year-old man from Louisiana. Even though he is the oldest among the three, he was relatively healthy. This only lasted under a year and I was never really worried about losing him. 
The third (current) one I'm in right now is a 7-year relationship with an Indian man who is 51 now. He is overweight, has obstructive sleep apnea, and several related lifestyle disorders. He also suffers from low-self esteem, anxiety, and depression. He finds it difficult to take care of himself and his health is a constant worry.
In the last year, I lost both my parents. Even though I live in a different city by myself, I was by them in their last moments and had to attempt to resuscitate both of them. My mother's death was accidental (literally). I was in the next room and I heard some utensils fall. I called out and did not get a response, and rushed to see what was going on. I heard some strange sounds and couldn't figure out what was going on. Then I found her gasping on the kitchen floor. She died within minutes while I was resuscitating her. 
Now what this has done is that each night that I spend with my current partner, I am worried about finding him dead next to me. Often times I wake up earlier than him, and I am forced to check on him every now and then to ensure that he's still breathing and not dead. 
I know this might sound really dark and pessimistic, but that's how I am. I know that I could lose him any day and nothing is really under my control. So I try to do my best to enjoy the time I spend with him, despite the fact that the going is tough because of several issues and differences in opinion. 
I don't know if all this helps. But I thought I could at least give you someone who you probably can relate to.

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