He ran up the stairs, breathing so fast

[alpacinou]

Is this correct and natural?

He ran up the stairs, breathing so fast he felt his lungs could explode. His heart was the same, hammering in his chest. As he approached her flat's door, a wave of doubt ran up and down his body, his steps became less certain. It's too soon, it's not the right way, she will say no, wait until she is ready. The words swirled around in his head in a chaotic, melting mess. Like most of his life, doubts were clawing at his heart, preventing him from doing something that could make him happy. No, not this time, I will do it. What will be will be. He grabbed the ring's box from his pocket and opened the flat's door to find her naked on the couch with another man.

 

[emsr2d2]

Is this correct and natural?

He ran up the stairs, breathing so fast he felt his lungs [STRIKE]could[/STRIKE] would/might explode. His heart was the same, hammering in his chest. As he approached her[STRIKE] flat's[/STRIKE] flat door/the door of her flat, a wave of doubt ran [STRIKE]up and down[/STRIKE] through his body, and his steps became less certain. It's too soon, it's not the right way, she will say no, wait until she is ready. The words swirled around in his head in a chaotic [STRIKE], melting[/STRIKE] mess. Like most of his life, doubts were clawing at his heart, preventing him from doing something that could make him happy. No, not this time, I will do it. What will be will be. He grabbed the [STRIKE]ring's[/STRIKE] ring box from his pocket and opened the [STRIKE]flat's[/STRIKE] door, only to find her naked on the couch with another man.

See above. Remember that with things like "car door/flat door/house door/ring box", we use a compound noun, rather than the possessive.

 

[Tdol]

How about a verb like gasping?

 

[emsr2d2]

Logic issue - the door to most people's house or flat isn't openable from the outside without a key. Are we to believe that she was inside her flat, having/about to have sex with another man, and she didn't think to lock the door? Or are we supposed to assume the writer had a key? Either way, it's unlikely that he could simply have opened the door from the outside. If he's about to propose to her, it's reasonable to think that he has a key to her flat but, if that's the case, I'd say that he unlocked the door before opening it.

 

[alpacinou]

Logic issue - the door to most people's house or flat isn't openable from the outside without a key. Are we to believe that she was inside her flat, having/about to have sex with another man, and she didn't think to lock the door? Or are we supposed to assume the writer had a key? Either way, it's unlikely that he could simply have opened the door from the outside. If he's about to propose to her, it's reasonable to think that he has a key to her flat but, if that's the case, I'd say that he unlocked the door before opening it.

Actually, he had moved in with her earlier. He said he would go on a trip so the proposal would be a surprise. He has a key.

Do I need both unlock and open?

He grabbed the ring box from his pocket and unlocked and opened the door.

 

[emsr2d2]

Did he move in with her earlier that day or before that? If they've been living together for a while, you need to say "the door to their flat".

I'd say "He grabbed the ring box from his pocket, unlocked the door and entered, only to find ...".

 

[Tdol]

I had a first floor flat with an external staircase in London- you could pant your way up the stairs.