There's a difference between saying "I understand I'll probably get postnatal depression" and actually understanding the cosequences, for you and your child. Having a baby because your grandparents are dying is also not ok. I had one very elderly nana with Alzheimer's on my mums side, and a healthy grandma and grandad on my dads. I adore my nana, despite her dying when I was six and her being very unhealthy mentally and physically, I also adore the grandad I never met, I believe I did meet him before I was born, I feel very connected to them, because my family have never stopped talking about them, they will always be a massive part of my life. My grandparents on my dads side not so much, my grandma recently passed, I was devastated, but because I barely knew her, I didn't feel all that close to her, nor to my grandad.
When you have a baby, whether your grandparents and parents are around, your kid will know and love them, if you tell them.
As boo said, someone with depression doesn't have to never have children. But there are stages of depression, you're in a stage that has just accepted you will always hate yourself and need others to love you to make you feel better but you can't accept that they love you because you think you're crap so they should too. I've been there it sucks, because you have to muster up everything you've got and jump out, no one can help you do this. I'm in a stage where I'm trying to climb out but the bastard is holding onto my leg and keeps pulling me back, my OH is holding onto my hand pulling me out, it's bloody hard but I'm proud of how hard I'm fighting and I know I'll win. I wouldn't have a child in my state, because I know I'll get post natal depression, and I'll hate myself for it and I'm scared that it'll pull me back, but in a year or two or five, I'll be strong enough to have a child and know that I might get depression but I can fight it, because I've won before.
like I said, my mum had depression when I was a kid, still does, I think she always will, she did before she had us, she was always so lonely, because all she had was us kids and an adult needs adult conversation. It was hard, because we felt like we had to look after her in a way, it made us grow up quickly, it made us hide school trip letters because we didn't have the money and it'd only upset her that she couldn't give us those things. I love my mum, I just she could be happy, I don't want my kid to feel the way I felt.
stop trying to push life on, life is now, it won't be better in a year or five or ten, not unless you make it better, start challenging yourself, it's hard at first, but you seem like you like to think logically, so be logical, if you hate yourself, be someone you won't hate, THERE IS NOTHING STOPPING YOU, ONLY YOU.