There are a few common idioms or sayings that fit the situation rather well, though they do not mean _exactly_ “having so much [work] because you chose it yourself”.

There’s [***“You made your bed (– now lie in it/you are going to have to lie in it)”***](http://idioms.thefreedictionary.com/You've+made+your+bed), which means that the person you’re talking to has created their own mess, so now it’s their own responsibility to fix it.

Similarly metaphorical is [***“You reap what you sow”***](http://idioms.thefreedictionary.com/You+reap+what+you+sow), which compares the bad situation to a field—if you sow bad seeds, you’ll reap bad grain; likewise, if you ‘sow’ a bad situation for yourself, you’ll have to ‘reap’ the consequences, no better.

More direct and pragmatic, [***“You asked for it!”***](http://idioms.thefreedictionary.com/You+asked+for+it!) is used to indicate that someone should not be complaining about their current situation, since it is only what they themselves asked for in the first place.

Even more direct is [***“You brought this on yourself”***](http://idioms.thefreedictionary.com/bring+on+oneself), which does not mince words and simply says that the person addressed caused their own misery themselves.

**Note:** All of these could be considered somewhat rude—their intention is to be blunt. If you wish to be softer and more polite about it, I don’t think there’s a fixed expression available that fits. Instead, I would just phrase it in a natural, but polite, way; something like, “I’m sorry to hear you’re so caught up in work—but you mustn’t forget that you _did_ ask for quite a heavy workload, so you really only got what you asked for”.

An idiom that is a bit less rude would be [***Be careful what you wish for—you might get it***](http://www.usingenglish.com/reference/idioms/be+careful+what+you+wish+for.html) (or _it may come true_, or _lest it come true_; there are several variants), which quite properly warns someone not to want or ask for something without first thinking through what actually getting it will entail.