For a third amv, I think you're doing pretty good. You should see my third one, terrible.

Well, you're still learning a lot and I think that I shouldn't do a generic criticism on your video. But one thing you need to have in mind, is that is really important that you get a deeper theme before starting your work.
As you asked, I will try to help you the best way I can for you to improve your abilities.
I'll put my compliments first:
Every song has an introduction. Or at least, every good song. A clever way to take advantage of that is to use the introduction of the song to place the introduction of your video. And I must say you did it very well.
You started it with that image of them interacting, in a room. You hear her voice and she's introducing herself I guess. That is really good and it makes the viewer emotions flourish (even if they haven't watched the anime).
It's followed by the guitar sounds, with the two of them standing in an opposite direction to each other. The cogwheels kinda trap them inside, predicting that they are together in an uninterested way. Or maybe they're together, but still not at all. Or that the only thing they got is each other. Complicated. But it's an introduction, right? And that's the main thing. Some people use titles, some people use images, some people use both. But the important thing, is that the introduction leads you to the main theme of the video. We see images flashing, giving it a stylized and messy look and that is beautiful. We see a hand trying to catch the sun, which may symbolize the pursuit for something impossible to reach. Okabe's (I think that is the guy's name) face is also presented in the image of the hand trying to reach the sun. And we may connect it to him. What's interesting is that if that's Okabe's hand trying to catch the sun, we then see Mayuri trying to reach the stars. By that, we may connect them as opposites to each other: man and woman, day and night. But what's really good and interesting, is that the melody of the introduction has two "phrases" and this phrases are well connected with the subject. See, the first part of the melody that starts, ends with Okabe trying to reach the sun. The second part of that melody, ends with Mayuri trying to reach the stars.
Then, as the shooting star passes through her hand, we can hear the main part of the music . We can hear the bass lines gently touching our ears as the video also starts to get deep.
We can say that the main part of your video starts with that shooting star passing through her hand, making reference to the sun that Okabe was trying to reach.
As the main part of the video starts we can see now closer images of their faces, showing their expressions and making us get in touch with the feelings of the characters.
Now, the bad points:
Well, as I said, a good AMV is based on a good purpose. So it's very important that you try to find a deep meaning for your work before starting it. Try something as original as you can, but still between what we consider being pleasant and beautiful.
As I watched the video, I also noticed that although your video has a story, it's not put together in a organized way. And I'm not saying that your story needs to follow a straight line, some videos with non-linear stories are simply amazing and they, are organized in a way that you don't even have to watch the animated series to understand what's going on.
That's the most important thing I guess.
It's also really important to know that the story in the animated series IS NOT the same as the story in the AMV. You can't put a huge story inside a short video. So cut things out, exclude characters, exclude objects and everything else that doesn't matter!
I couldn't stop noticing that when Okabe's trying to call her, she then appears holding a bottle of soda. I don't even know why, but that bottle of "Dr. Pepper" appears taking away half of her face in the image. Maybe that's something that characterizes her in the story of the anime, but we shouldn't forget that the story in your AMV does not have time for that kind of stuff. Of course, it's still possible to create an amv where a Dr Pepper soda is relevant to the story but that, I think, might be a little bit complicated. Well, now let's suppose you chose that image because the "zoom out" it had could fit with the sound of the strings that proceeds the refrain. It's good, but you shouldn't do that if it's still adding non-meaningful subjects.
There's a part that really bothered me:
Around 1:35 you show the Mayuri in a cemetery, but the scene starts with a bucket. What's in that bucket that is so important? Again, don't be afraid of cutting things out for creating an AMV! You must have in mind the fact that not everyone watched the animated series, and you should do something that everyone could understand. Well, obviously, she's weeping about someone who's gone. Her face is hidden and all we can see is her mouth saying words. That makes us think that she's somehow hiding her sadness. Okabe stays beside her, almost crying. What bothered me is that you chose this scene to state that Okabe has made a decision to stay beside her. As the sentences of the song are shown beside him while he is weeping. That makes us think: Why would he fall in love with her there in the cemetery? As he's not in front of the grave, we could assume that he's not related to that person who's gone. He's hearing the words she's saying, and that's making him cry. But why is he crying by seeing her there? Well, something happened so that he would cry. Something happened so that he would feel her pain. You could use flashbacks, you could tell a story about his past there and why he fell in love with her in a cemetery.
The sudden use of words in the image it's used to amplify the meaning that those words have to the image that's also shown. While that image's got me thinking of: "I wanna take your pain away." Those words made me think of him making plans to have her for himself...
A good point I forgot to mention is that you chose the right moment in the song for the most intense happenings that end up leading to her death.
I just think you shouldn't end it there. If the lyrics of the song are about Okabe's feelings for Mayuri, you should show us a little bit of his pain and acceptance.
Another thing:
Look at 00:20
You show us Mayuri looking to the night sky.
Suddenly the image is cut to her looking down while Okabe puts his hands on her shoulders. Be careful with that! A romantic/sad/sentimental/slow video should look as smooth as possible.