I'm sad so let me make things sadder

Both needed more time, especially Uke. Kinda wished Uke & Seme had a recounter years later.

Either Uke is older and had actually fell in love with someone else and was loved back. And idk, Seme & Uke just talking about how they love each other but Uke knew Seme's real love will always be reserved for his "heart"

That Seme will always be Uke's "heart" but the man Uke is with now. Crying, Uke says how happy said man makes him. That that's the man he wants to wake up with everyday.

Seme wishes him happiness and there we see Seme slowly seeing how important Uke was, just constantly missing him but denying it saying it's because they were together for so long. It's different having Uke not here.

Then IDK
maybe it ends with Uke on his death bed, last days and the Seme being there to see him. Uke making jokes on how he's the old man, now. Uke thanking Seme for taking him in and everything he did for him, the bad & good. That by letting him go, he found love & happiness with someone else. Uke is ready to see his lover again after he dies. But he's happy it's Seme he gets to spend his last days with.

Maybe Seme breaks down and asks Uke to be his "heart", that not to leave him. But Uke calms him down, telling Seme not to give up on his "heart". That Seme's dream is one of the reasons the Uke loves him.
Uke pointing at Seme's chest, "I'll be gone, but here. My love will always be here. And even after you find your "heart". It will still be here. You'll always be my heart, Seme."

Then before Uke dies he wishes Seme to find his "heart" soon. That he wishes Seme & "Heart" are able to experience the same happiness Uke felt with his lover.

I just need Seme to be slapped with angst hard after Uke dies.
That Uke being his "heart" doesn't matter anymore. That Seme even starts hating himself for being so obsessed with finding his "heart" he lost Uke. That if he hadn't, him & Uke could have been happy & in love. That they would have had so many happy years together. It's knowing it's far too late for any of that to happen. That has Seme sobbing.

Extra could be Seme meeting a reincarnation of Uke and saying idk, "I've found you, my heart."

(Do these vampires live that long? I honestly forgot. Either way here's my sad version (￣∇￣")