"If Gwen really loved Miles, why didn't she try harder to make their relationship work? Why did she give up on everything? Why didn't she tell him she loves him? Why didn't she tell him about his dad dying? Why didn't she decide to help him earlier? Why did she let Miguel and Jessica brainwash her? Why didn't Gwen try to save her own father in the first place?"
If you look at the story from her pov, she's clearly going through a lot and is an emotional wreck because one traumatizing event happened after another and so on... So far, her own life taught her that she might never be truly happy and constant suffering drained her and made her think that she just won't get a happy ending, ever. She thought that she could save Peter but she couldn't, she thought that her father would understand her but he refused to, she probably thought that she would get to patch things up with him one day but then she learned that he was meant to die in the near future, so she was probably thinking "Wow, there really is no point for me to try anymore. Maybe I'm just not meant to be happy."
Miguel found Gwen when she was vulnerable both emotionally and physically. Betrayed by the only support system she had and couldn't disagree with Miguel and go back to her own dimension and end up in jail, so she just went along with whatever Miguel decided was right.
So why would she try to date Miles? Why would she try to warn him about his father's death when, to her, suffering is inevitable and everyone is supposed to go through that, right? Why would she continue to talk about Miles to Miguel so Miguel would let him join their team, when Miguel already decided to not let him in and that's just another thing in her life that isn't going to work out?
To Miles that's all personal of course, to her that's just the way things are. Don't forget that she was surrounded by spider people (and some of them were her actual mentors whom she looked up to and the only support system she had at the time) convincing her that that's the way it should be.
So losing Miles is difficult for her, but she already got used to losing and she was in a place where she didn't have the power nor motivation to even try to change anything for the better. Only when Miles showed up and started resisting did she suddenly wake up and want to at least try something, anything. Then her father presumably avoids the canon event AND he's feeling sorry for hurting her and she's like "Wait so good things can actually happen to me? And if they can happen to me, then they can happen to others too? So maybe I can help Miles."
I promised myself that I will stop oversharing, but I want to explain why I understand her. I've been in her shoes for years, both physically and emotionally, and being stuck in place while the bad things never end can make you feel so tired and so unwilling to the point where you're thinking "My life sucks no matter what I do, so why should I try at all?" At that point you just want to barely survive, not win. I could've been happier a long time ago and accomplished things I have accomplished now if I had just taken risks earlier and tried harder, but I didn't have the means. When I finally found my way, it took me a while to catch up with it emotionally and truly want to try harder. Because sometimes a person's hands are tied even when they want to fight, and when their hands get untied, it takes time for them to regain their mental strength.
That's just how some people react to trauma.
