I don't have any experience in medicine or expertise in writing personal statements, so take this with a grain of salt.

Overall, I'd say it needs to be more specific. In the first paragraph for example you should talk about some specific research you worked on, and how that experience makes better prepared for medical school/ to be a doctor. Even though you don't want to go into research, being specific helps differentiate you since everyone is going to write that they did a bunch of research and liked it. The same goes for writing about empathizing well. Everyone can say that they empathize well and like working with patients, but it's much harder to give a specific example backing this up, so this could set your statement apart from the rest.

In the third paragraph, it's good that you give some evidence that you work well in high-stress environments. I'd not write "Few people would find such a workload appealing, but I relish it." since there are probably a lot of doctors who do thrive in this environment. The rest of the paragraph after that sentence is also problematic, since it makes you sound like a workaholic or someone who will not react well mentally under certain conditions. You shouldn't write "I feel slothful and unproductive" or "I am disgusted with myself" because it will make whoever is reading associate you with someone who is slothful unproductive and has self esteem issues. Also "I wouldn't be happy with myself" in paragraph 2. Try to rephrase those in a more positive way; working hard and helping people makes you happy/fulfilled instead of not working hard enough or failing to help people makes you miserable.

The part about your reaction to your girlfriend's illness is probably OK since it's an example where you show that you can empathize well with and help sick people.

The last paragraph seems a bit redundant, you could probably pare it down to leave more room for specific examples elsewhere. It would be helpful if the first paragraph started with a better hook since "I've always wanted to be a doctor" is probably the most common and least original way to start SOPs.

The only suggestion I would make to this statement would be to switch the first and second paragraphs. I'm no professional reviewer, but from all the proof-reading I've done for friend, I think the "ever since I was a kid" intro is one of the more common ones, and therefore not as likely to pop out as the more immediate story about the tumor. You'd have to rework the sentences a little, like "I never truly realized on a personal level how powerful a connection between doctors and patients is until last fall, when doctors discovered a pilocytic astrocytoma in my now-girlfriend’s spine." You could also go for a suspense thing and just say it, but that might be too much of a gamble.

I'm not sure medical school admission boards would acknowledge this anyway, but I thought this might be of interest regarding the first two paragraphs.

Also possibly of interest on a similar topic.

I know that wasn't what you were asking for, and I don't mean to the bearer of bad news, but forewarned is forearmed.