This sounds like a YA version of THE HEARTBEAT HYPOTHESIS, a book I absolutely adored. So you've caught my interest right away. The query is largely well done, but I do have a few questions.
1. What has Sophie been doing for the past year? Has it all been rehab/recovery (I don't know anything about lung transplants.) Or was there some incident that made her see the need to live life to the fullest, when she wasn't thinking about it during that first year? 2. How old are Sophie and Jason? This is YA, meaning she could be as young as 14. That leaves me with questions about how they're traveling all over the world together, apparently unsupervised. Tossing her age into the first line would help with that (assuming she's 18). 3. What is it about Sophie that makes Jason change his plans, even temporarily?
For me, honestly, four comp titles is too much. I'd leave it with Seven Pounds meets The Bucket List, because If I Stay is a massive bestseller.
Otherwise, I'd just do a scan for word choice and tightening the language. For example, I don't know that you need to say he "secretly plans" to kill himself. The secret seems implied. Don't tell me Sophie thinks she's showing early signs - just say she's got the signs. It's more compelling that way. Things like that. But overall, this sounds like a great story and the query is good.