I was also in my mid twenties when my mother died of cancer in the span of about nine months from diagnosis.
It was a terrifying and overwhelming time and I felt completely adrift as there simply wasn't anything I could do to change the fact that she was dying and there was no other choice than to accept the situation as it was. It's something that continued on in grief (it's v. normal) until I accepted that I'd done all that I could for her and she'd be the first one to say I'd done far more than necessary. We can't change the past but we can influence how we feel about it in the present. Finding some sense of control and acceptance in it all helped me greatly.
The day she died I woke up with the thought that this was the day. It was a very strong feeling, the decline is visible she was becoming more and more lethargic, losing desire to eat and drink, turning yellow from liver failure, etc. There was no sense in denying the inevitable. She was still lucid the night before, I can't even remember what we were chatting about but that morning she seemed very far away, as if it was just the shell of her.
Her last hours were upsetting for me, she appeared to be in a lot of pain even though we were assured it wasn't the case. The body tends to fight death till the very end. I'd been with her every step of the way of her disease, I'd lived the pain with her for months but this really broke something inside of me.
As hard as it is to imagine, her death came as a relief to me for she was no longer in pain. That's all I cared about at that point, and I was genuinely happy for her release from it. Seeing someone you care for so deeply in pain is something I wouldn't wish on anyone, I could only care about my own loss later. Her death was about her, not me.
After, is just as daunting as where you are now albeit in a different way. It does get better, but it never really goes away. There will be days that are unexpectedly hard while others turn out to be a breeze. It stops being an open wound that renders you useless at the smallest prod. Peers are tough to deal with at this age, the concept of losing a parent is so foreign to most that they simply don't know how to cope. People will surprise you, in good ways and in bad. It makes a lot of things suddenly very clear and establish a level of maturity you didn't know you were capable of.
It's been a few years on now, it can still catch me unexpected at time. The words of others who haven't been through it are difficult to comprehend sometimes, if I mention it to someone new the automatic response is always, 'I'm sorry.' For whatever reason it always feels like they're saying sorry for bringing it up. I always answer, 'Don't be, she lived a full and happy life.' because she did and I'd much rather remember her at her best than her memory being defined solely by her death.
I'll pass on the best advice I received for after, make your bed as soon as you wake up and set out to do/achieve at least one thing each day. It doesn't matter how small or frivolous.
Good luck OP and feel free to PM me if you're ever in need of a listening ear.