I went through more or less the same thing a few months ago - found out my bloke of 4 years had met someone else and was (kind of ) seeing them behind my back.
I ended up txting her - calling her rude names etc. Initially I felt better but then horrible. Also did the revenge thing with my ex - nothing worthy of a custodial sentence and very cathartic at the time but again it didn't help.
What REALLY worked was blubbing over friends and doing all the stuff I'd wanted to do for ages. You're bound to think about him loads - I know I did. You'll think about her too and be angry - it took me months to get over the fact that he'd run off with someone the complete opposite to me (and marrying bizarrely enough after about 5 months of knowing her - I thought I'd be dead upset but I can't even be a***d to think about it)
Anyway a few nasty moments ensured but I ended up meeting another fella - a real sweetie - and I can honestly say I've never been happier. I'm not saying you need to find a bloke, but go out and do stuff and you WILL feel better soon.
In the meantime I'd send him a measured, sensible email asking for the money for your holiday back - its horrible I know but you'll come out of it feeling stronger. Failing that put itching powder in his boxers : )