I don't believe you want suicide. I believe you want CHANGE. Only, having tried so many ways in the past to make some change happen, and it didn't work, you feel death would be the only way to change things.
You sound tired. You sound like someone who has pushed a truckload of right beliefs, right behaviour, right ways, uphill for a long time, seen no rewards for that, so are giving up? You see people who do wrong 'get on' in the world, and then you, who has lived by right ways, are doing badly in work-life and social life?
Is there anything in this world that you feel for? Even the smallest thing....? Could you love a dog, for example? Could you get into astronomy, or writing stuff, or painting, or anything at all, whatever it might be? Never mind for now about relationships, just think "Is there anything, whatever it might be, that I am at all interested in, and never mind anyone else"
I kind of feel that there IS something living inside you, which COULD be passionate about something life has to offer. You just haven't had that button pushed yet. And your looking after your Nan was a kind and selfless thing to do. But I know how mind and energy-consuming that can be: caring for an older person, especially someone with Alzheimer's developing at the time you were looking after her. You wouldn't have had much time to think about yourself, your talents, your needs. You wouldn't have had much time to discover that something deep inside you can be interested in SOMETHING, give energy and passion to SOMETHING whatever that may be.
I really feel you believe yourself to be such a failure, stuck in such a rut, that the only way to change that is by dying. Maybe that is not true at all! If your Nan could speak to you from the 'Other Side', what do you think she'd say to you? Probably "Thanks....thanks for giving so much to me...thank you for helping me and being my friend as well as my grandchild. You really made my last years happier."
That wouldn't be the eulogy of a failure, now would it?