Hi there! We have all been there, I remember freaking out about the surgery too.
I had mine back in November, and now I feel like it never happened. Although, I thought I was pretty athletic before the surgery, now I feel that I wasn't. Mainly because I feel so much stronger now (and it has only been 4 months since the surgery).
What kind of OHS are you going to have? I had a minimally invasive OHS, which meant much more pain initially, but a faster recovery afterwards. If you want, you can read my story here: http://atrialseptaldefectsupport.org/forum/topics/open-heart-surger...
What helped me the most was being curious about what was happening. I kept focusing on "oh, I wonder how they do this" or "oh wow, I wonder how this works" as opposed to focusing on "OMG, I'm having a surgery! It's gonna hurt!" And when you wake up in the ICU, don't think about how painful it is. Think that from that moment it will only get better! Feel free to ask any questions, and good luck with your surgery!