Without knowing your personal circumstances it's hard to comment on certain aspects. However, if you are up to your neck in debt, have recently returned from a expensive holiday, are saving for house extension or kids university fees looming, and if the job could not have been rearranged for a day or two later, and the client is a major contributor to your workload then its justified that you were trying to sustain income both now and for the future to continue to fulfil your financial needs and went to work.
The information given lends itself to believe that for you working is your hobby, so an opportunity came up for you to indulge in your hobby, you took the opportunity.
One way to look at this would be to turn it on its head.
Lets say you had huge job on where you need a second person, the job is in a very open public area, you needed a second pair of eyes. You had arranged with your gf to be there, keeping an eye on general public and watching your back. Nobody else could have replaced her as the job didn't financially justify a second man because you had to be competitive, nobody else was available anyway that you would trust.
The night before the job your gf gets a phone call "hey, fancy going to a spa day tomorrow," your gf mates mum is not able to go and gf has been offered a day at a spa free of charge. She doesn't like stump grinding anyway so she says yes, turns to you and says you're on your own tomorrow, enjoy your day I'm off to the spa.
How would you feel?
There are many situations in life where we have to do things we don't want to. Where we can self justify"letting people down". Ultimately, you let your gf down, she will have some kind of scar from that. Human nature in some people will lend to their thinking that when events like this occur, they can't rely on some people. Only you know how good or bad this had gone down within your household. Only you know the extent to which you need to go to, if needed, to make things good, to explain and show your understanding but also how much the relationship means to you.
Good luck, and of she is a keeper, don't under estimate the extent your apology may need to extend to.