What is the significance of 45 Years Movie?

As stated in Vogue:

Haigh, who adapted the screenplay from a short story by the British writer David Constantine, leaves things maddeningly, gloriously vague (not least of all in the film’s much-discussed last scene, at the anniversary party, in which the virtuosic Rampling allows decades of hurt, anger, and confusion to surge silently across her character’s face). And as a result, 45 Years haunts long after the final credits roll.

Note that this ending does not come from the short story the movie is based on:

45 Years is based on “In Another Country,” a short story by David Constantine, and it borrows not just a premise and passages of sharp dialogue from its source material, but also the barbed efficiency of the short-fiction format: At a spare 95 minutes, this is a film of no wasted scenes or unnecessary subplots, stripped down to something tough and focused and vivid. Yet the anniversary-party angle is all Haigh’s invention, and he uses it to devise a phenomenal new ending, one that — like the jaw-dropping finale of this year’s Phoenix — draws its power from close attention to behavior and minute shifts in body language. At the center of it is Rampling, all eyes on her, showing us everything that’s going through this woman’s mind. But even when it’s just her in the frame, when the camera has come in for a close up, there’s someone else there just out of view—the invisible third party of her marriage, the ghost over her shoulder. She’s still there. She always has been.

He simply loved another woman more than her 45 years later. He for 45 incorporated parts of the woman into their lives from everything to her perfume,the books she read and her memory. Yes it seems he loved his wife but she wasn't the love of his life he longed for and seems nowhere close to his great love. He himself said he would have married the other woman before her. So at the end that was a face knowing her husband had lied to her for many years and she lived with a man that only half loved her. Maybe the face of a wasted life and you can't get back 45 years.

I think she is trying to suck all the pain in. Thinking "so for 45 years, you have loved me but with her still at the back of your head?". IMO, the ending scene is just so perfect because they are depicting a happy anniversary party but her emotions are obviously the opposite.

When you watch rampling's face as she is paying attention to the lyrics of the song and finally realizes just what they are conveying, in reference to geoff's real meaning and intention for choosing this song (whether he's even fully aware of it or not), when the last word is sung and he happily sings along and raises both of their hands joyfully into the air, her very intentional letting go and jerking her own arm deliberately and forcefully down to her side along with the stunned look in her eyes...in her face clearly explain the entire emotion of the movie. By the way, the use of the sound of the photo slide clicking to accompany the credits is just a wonderful little clever touch, foreshadowing in the opening credits and verifying it's significance in the end credits.

"Smoke Gets in Your Eyes" was the same song chosen for their first dance 45 years ago, but perhaps this was the first time Kate was closely listening to the lyrics. I believe all the songs played or named in the movie have some sort of significance with regards to the story.

Also looking at the photos at the beginning of the party reminded her of Katya's photos she was looking at in the loft, realising that the decisions such as holiday destinations were taken based on Geoff's memory of her.

Her 45 years of love was under question. Was she also chosen because of her? She had the same hair colour, and even their names are quite similar.

From what some others write you might think this was something akin to Rebecca? Kate realized something she had never been able to put in words. And she wouldn't have it. Can she forgive him, ever? It appears she might not. Is she reasonable? Of course not. Why should she be. But then there is the reality of it.