His thoughts are a mix of happy or meaningful memories, ones he feels guilt for, and symbolic ones. The list is in order of appearance.
Tommy and Grace kiss in the afterglow of their first time. A moment of deep peace and happiness for Tommy, when the horror of his war memories were silenced.
2. Running to bring Ruby to the hospital because she was so sick. A guilty, sad memory.
3. Ruby's spirit comes to tell him not to die. A happy, meaningful memory.
4. Ruby playing during the scene where Tommy must kill his black horse. A happy memory.
5. Grace turns around after closing the Garrison, when he is about to ask her to go to the church where they have their first kiss. Honestly I think they used it because she looks really beautiful here, almost ethereal in bright light., but it also could be symbolic for her turning around to see him there- a prediction for when they meet again. Either way, it was a happy memory.
6. Tommy plays with Charlie after Grace's death. A happy and sad memory.
7. Grace confessing in season 1. The memories here are solely tied to either happy, guilty, or symbolic memories or ideas, so this certainly can't be about feeling betrayal from the original moment. I believe it is either guilt for not having forgiven her at the time or a recognition that he will be confessing his sins, apologizing for the mistakes he made, to her when they reunite.
8. A shot from when Tommy is preparing to go to the races, water drips from his face that looks like a teardrop.
9. The first obviously symbolic picture, the eye of a black horse with a setting sun in the background- the final death of Tommy's dark side.
10. More Tommy prepping for the races, using a straight razor against his face.
11. The window of the car where Arthur was killed by Tommy, his cap is visible from where he slumped forward motionless after death. A guilty memory.
12. Tommy and Grace dancing at their wedding, this is the moment where Grace was about to ask Tommy to not let anything happen that night and to leave the bad business behind. Since he didn't agree to do that, this is a guilty memory. It also recognizes that this was the moment he signed her death warrant, metaphorically. Pairing the Arthur scene and this also supports that.
13. Tommy cleaning himself for the races. Symbolic of cleaning himself in preparation to meet his family.
14. Grace asks him if he wants a happy or sad song for her to sing. Although he asked for sad at the time, I'm sure this is a happy memory for him now.
15. I could not find if this was from a specific scene or if it was something new, meant for symbolism. I thought it was another race prep scene, but I couldn't find it there- when I rewatched. It appears to be a nude person standing in front of a bed, their hair is short so it seems likely that it is meant to symbolize Tommy going to rest, everything bare now.
16. Polly at his wedding, looking concerned and worried because of the bad business going on. Likely reflecting her maternal role for him.
17. Grace putting on lipgloss in preparation for the races, although they changed the lighting. Tommy would have no memory of this, so it is meant to be one of the symbolic thoughts. It shows that as he is preparing to meet her, she is preparing to meet him too, at least his mind hopes she is. The lighting does make it similar to the elements of the dark horse picture and the final shot of Ruby waiting for him. If the horse had a setting sun, I think this is meant to symbolize a rising sun, and in Ruby's as well.
18. A scene of a gun, another one from the Cheltenham race prep and I think it belongs to Grace. It likely represents the presence of guns in his life, a source of guilt.
19. A scene from season 1 episode 3 where Tommy cheers up Arthur after he gets upset that it was Tommy running things and not including Arthur in what's going on. He then buys the Garrison for Arthur. A happy memory and one of the many that cemented their relationship as ultimately loving.
20. The second one from Grace's confession. Again, this has to be connected to his own desire to confess to her.
21. Smoothing his hand over her back during their first time, something he relished doing when they met again in season 2. A special memory.
22. Grace puts her hat on, ready to go to the races. Ready to meet him again.
23. A memory of Tommy preparing his gun when getting ready for Cheltenham. Mainly I think these are meant to show his regret that guns were so involved in his life.
24. This seems like a new scene since the figure is wearing the same outfit she wears in the film. This is meant to show Ruby waiting for Tommy in the afterlife, or as he imagines it will be. She had stopped him from killing himself before, but now it is finally time.
25. Tommy washing himself one last time from the Cheltenham prep. This symbolizes him cleansing himself before he sees his family in the afterlife.
This montage restored my faith in Peaky Blinder metaphorical writing.