YANBU @hanketypankety For someone who has never been alone, the thought of being alone is quite scary. As I said on a thread earlier, life is a struggle financially for many single women, as the vast majority are NOT on £100K+ per year as many on mumsnet claim they are.
I have to admit, and this is JMO, and just my thoughts and musings, I sometimes wonder how people cope when they're alone. No partner, and no children at home anymore. No-one to help if they're sick. Just them and the four walls that surround them. I have been with DH for 33 years, (married for most of them,) and have 2 kids who left home 5-6 years ago.
I lived with my parents til I was 20-ish, then moved into a flat with a friend for a year and 3 months, until she moved out to live with her boyfriend. I was alone then - no boyfriend at the the time - and stayed for 7-8 months til the end of the tenancy term.
Even as a 21/22 y.o. I found it really lonely living alone. I came in from work - I was alone, I went to the pub to meet friends for an hour or two and came home - I was alone. I went to see family for a few hours and came home - I was alone. I went to bed - I was alone. I got up and had breakfast - I was alone.
I got so down from the loneliness that I moved back in with my parents. Aged 22. Moved out 2 years later to live with my boyfriend I met at 23, married at 25, and have been married to now for 30 years...
Purely anecdotal, but I found being alone a struggle mentally. I also struggled financially, and I was on a reasonable wage. Being with someone, and sharing the load, and the finances, and just having someone to TALK to, and pours your woes of the day on, and have a laugh with, or watch a movie with, is SO much better than being alone. IMO. As I say IMO. This is just me...
A couple of years ago, my DH was in hospital for 4 days, and I missed him terribly, and was SO lonely. It wasn't so bad when he was away (with work for example) when the kids were at home, because I had them for company at night and in the mornings and the weekend etc... But they left 5 or 6 years ago, so I was alone in the house.
I don't know how people manage long-term being alone. How do they cope if they're ill? How do they cope financially?
As I say @hanketypankety YANBU. I fear being single too. If other people are fine and dandy being alone, then good for them. Wouldn't do for me. Maybe I am fortunate to be in a good marriage, (30 years now,) and not be in a bad/abusive toxic marriage, and it IS understandable that people want to leave very bad marriages.
But I think if it's just an average/bland marriage where you're more like friends/housemates than lovers, it's understandable why many women stay. Many women are better off in a boring marriage to a man they are not in love with, but get on with OK, than they would be if they left and tried to make it alone. IMO.
As I said in another thread earlier, the dream/fantasy scenario that some mumsnetters come up with, of a middle aged woman leaving her husband and starting a wonderful new life, gaining a glittering new career on £75K, getting a lovely shiny new house, and acquiring a whole new bunch of friends, is just that: a dream and a fantasy. Never happens. Not for 99% of women anyway. The only time it happens regularly, is in novels. For the vast majority of women, there would just be loneliness and poverty...
OP, only YOU can decide what to do, and what is best for you. 
Only YOU can decide if your marriage is bad enough to leave it.