I lost by quite a bit. Can anyone help? Thanks

Edit: Yes i should exchange h18 first. I also added few comments

17: Joseki is H18. He still kills them of course, but now moves like G14 are sente, and he has to watch a shortage of liberties which allows you to connect with D15 and E15.

27: This makes C8 just too convenient for him. Maybe E3?

43: Sorry, but B10 is an ugly move. It may be tactically justified, but I'd still play D8 or tenuki just for aesthetic reasons.

47: If you had played H18, this would be sente to throw in at E17 and kill. So he would have to defend against that, and you then would connect with C13.

48: Ouch! He disconnects you in sente. I think you are screwed here.

51: I'd seriously consider abandoning them.

I think that much of your mid-game losses stemmed from the fact that you had the floating group in the middle that you always had to keep an eye on. Move 17 adversely affected the whole game.

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I actually like Joaz' comments. Some further ideas, to be taken with a grain of salt:

10: Did you consider E17? It looks interesting to me

51: I would exchange B12 to destroy an eye. But anyway the game move feels a bit thin to me, I'd prefer to play F12 myself (I don't think you can sacrifice these stones and win).

53: This allows white to really get out. Besides, there seems to be a big weakness left at H13. Do you have an answer to that?
Maybe tsuke at H10 is better than ? At least white should not walk out this comfortably, after 53 black looks kind of screwed indeed.

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My name is Gijs, from Utrecht, NL.

When in doubt, play the most aggressive move