I'm not trying to be offensive, but it may happen.
IMHO you are actively contributing to the stigma of PTSD. Hiding a diagnosis is by fact lying. PTSD is not what is wrong with someone. PTSD is about what happened to someone.
As you said in your OP you are 17. This is still very young, and it's good to have dreams. Please don't let me discourage you from pursuing your dreams. Society has probably done enough of that to you already. But PLEASE be realistic of your career options. I don't know you, or your situation and severity of your conditions. Maybe you could have a successful career in the military or as a police officer. I don't know.
I'm retired USN (20 yrs). I was in Gulf one and two, with two cruises in between where our battlegroup fired on live targets in Iraq often.
If you enlist in the military under the circumstances you detailed above, you will be guilty of fraudulently enlisting, a crime. If you have PTSD, you have PTSD whether or not your T is avoiding writing down the words, and is in breach of the guidelines that govern their profession by assisting you to commit fraud. You must declare during the enlisting process any condition that might affect your ability to serve. The military is life and death - that is it's nature.
I hate to say it, but the stress of extended deployments, particularly if your not totally "stable" will increase the odds of you not being deployable at some point. If this happens while on deployment, you will drastically affect the person, and the persons family who has to deploy on no notice to replace you. You will affect the people you work with, because once on deployment people settle into a routine, jobs get assigned, people develop "specialties". Then you get shipped out on no notice and everything is in turmoil. Moral is affected, the efficiency of the group is affected. Your country has to spend money to ship you back, and ship out your replacement (a lot more than you think because it's not just the transportation, it's all the admin of coordinating the transportation, security clearance checks, reshuffling a new person into the job of the person who is replacing you, etc...) I could go on, but you should have an idea know of what I'm trying to say.
I have a very, very low opinion of people who fraudulently enlist. They are criminals who endanger the lives of their comrades. They are a security risk. They are unworthy of trust.
As for police work... Just being a police officer can cause PTSD by it's self.
I was on an anti-terrorist/counter terrorist team for 2 1/2 years in the navy. A collateral duty (part of the job) was reserve military police officer. I worked with DOD police, the local cities police (several local cities), Sheriffs, Highway Patrol, and assisted the Secret Service (as expendable outer shield usually) in protecting President Reagan when he transitioned to/from Airforce One while going to/from his ranch in Santa Barbara and local functions.
With the above experience, I can say with confidence that someone with PTSD should avoid in most circumstances becoming a police officer. If you have, or are borderline PTSD, you probably will be destroyed by the stress of the job unless you join some little country town that the worst thing that happens is "Bubba" getting drunk on Fridays. Even then, do you really want to have a gun with you, walk into a situation where people are trying their best to piss you off, and even physically attack you. Will you have the presence of mind to function, or will you suffer a PTSD episode? Are you willing to endanger the lives of your fellow officers in a crisis? If PTSD is contributing to an already stressful situation will you be able to keep situational awareness? Keeping situational awareness is critical to the job. The more stress you are under, the more likely you will develop tunnel vision on the person that is the biggest perceived threat. That person may not be the one that kills you and/or a fellow officer(s). Fact: most police officers that are shot, are shot with their own gun.
Just some things to think very seriously about. Best of luck in your career of choice.
