^ This. I think one of the things that Lavellan (at least my Lavellan anyway) needs in order to move on is closure. An explanation from him. Why did he suddenly end the relationship? Why did he just leave? And she never got any. 
I reckon it's just human nature to be more plagued by anything that's been left unfinished or uncertain. Of course you could argue that he dumps you - and that's as clear a moving-forward signal as you need - but as people point out he does it in a "I can't tell you why, and now I'll back away while giving you longing, tragic looks" sort of way. Maybe it's not rational or healthy, but it makes sense to me in terms of real experience.
I understand both of you, but that was me 10 years ago. Now having gone through the experience a few times, I have learned that you hardly ever get a reason. And if you got it, it often would not actually help you. When I was younger I needed closure so badly, but over time I've learned that this was not good for me. And the more often I experienced that history repeats itself, the less I needed this closure. I would still prefer to get it, but I would not diminish myself for it anymore. Waiting months for somebody to tell me why he left me? Especially if that person never tried to reach out and help me understand? My pride would not allow that anymore. Usually that person will not give you what you want (and more often than not it is not even about closure, even if we tell that ourselves, it is the desire to get a change to put things right, to convince the other one and make him see that there's a chance after all) and so you just risk looking like a fool.
In real life there is usually only one reason, which is "you are just not enough to want to stick with you". What this "enough" is, is almost irrelevant. Not pretty enough, not funny enough, not smart enough, not open-minded enough, not whatever enough. What does it matter in the end? You are what you are, you cannot (or should not) just change because somebody else would prefer you were different. All you can do is find somebody else who does want to stick around.
Lavellan's case would be a bit different if she knew he's that infamous ancient elf, who had no other chance but to leave. But she does not know, so neither I nor she can consider this in the thinking process. All she knows is he left her and never got back in touch. He knows where she is, never tried to send a message. That he has good reason to do so she might assume but cannot know.
But that's why I asked, because how people handle feelings and how long they stick to somebody who left is so individual. I know people in real life who couldn't get over somebody for years.