There's answers already here, but some seem to disregard systems and laws in place in The Netherlands, so I'll give it a shot.
First up, you're not required to tell your work anything, unless you're taking regular time off for medical reasons. Medical reasons could be seeing a doctor, picking up prescriptions, seeing a psychiatrist, anything.
If/when you're taking (needing!) regular time off, then you may have to discuss your situation with the in-house doctor. Would also advise this, just to have someone close that's aware of what you're going through, but that's me.
Things to look at:
First up, bit of legal crap. If you're worried about legal repurcussions, like quoted below, you should:
- Contact Juridisch loket for advise (free)
- Check, or make sure to get, "Rechtsbijstandsverzekering arbeidsgeschild" - which is an insurance covering any legal costs if/when you have a legal conflict with your employer (let's hope it doesn't come to that, but the wait time until you can use this insurance is usually 3 months, so get it if you haven't - about E10,- a month, depending on coverage, provider and "own risk")
I am afraid that if I state that to my employer, they will reduce my compensations. I know that legally they can't affect my salary, but we also have bonus and the 13th month extra.
About that, the law is that an employer must have an insurance to cover 70% of your salary if absent for extended periods of time. Some bump that to 100% with an additional insurance. You can also get this 30% covered yourself (costs < E5,- a month). An employer is bound by law to pay your 70% of your salary for 2 years if you get sick that long. More info
Next, the health things.
Go see your own GP ("huisarts"). They will refer you to whomever you need. That way the cost is covered by insurance. Check with your health insurance how much of what is covered though (e.g. 20 sessions with psychiatrist).
If/when you're diagnosed as depressed and need to see a psychiatrist, you may wish to inform the in-house doctor to contact your GP/doctor or psychiatrist. That way, the in-house doctor may vouch to management/HR that you need regularly scheduled time off for medical reasons (you need not tell them why and the in-house doctor is bound by law that (s)he may not divulge this either, not even to their employer).
About your salary:
You have this black on white, written, slam-dunked as a contract. No worries there. Come renewal time they most likely will not reduce your salary and of course you may always negotiate yourself. Don't know what the law is when it comes to a new contract though, that you should check yourself (ask Juridisch Loket for example).
Is the 13th month also contractually written down? Or is along the lines of "added to contract house-rules" where it's mentioned as something of a "performance based reward". If part of the employment contract: no worries. If a performance bonus, then you may wish to discuss (after above things like seeing a doctor/psychiatrist and other options of outside help) your situation a bit, only to the extend your comfortable with.
So, lots of info.
Maar kerel, succes en beterschap.