Culpability–how much you are responsible for a choice–is affected by whether or not an outside force pressures you or forces you to act as you did. To the extent that your physical state is out of your control, you are not responsible for what your physical state forces you to do, even when the physical state shows itself in your emotions or in compulsive thoughts. Your responsibility is to “head it off at the pass” to the degree that you can, as you would if it were an actor outside yourself trying to compel you to do the wrong thing. You ought to be taking the care to keep yourself safe that you’d expect yourself to take if you knew someone else was bent on hurting themselves or hurting a third party.
You know that cutting yourself is not something that your doctor would consider a constructive way to deal with your stress, either, right? The relief is real and yet it is also illusory. Cutting yourself does not accomplish a positive good, but damages your body and leaves you open to infection. Work diligently with your doctors to find a way to cope with your stress that is not self-damaging.
As for tatoos, the Church does not hold that putting permanent color onto your skin is mutilation, per se, or that it is a sin. Likewise, piercing your body to allow for hanging jewelry on yourself is not automatically considered self-mutilation. Certainly any rule for decorating generally applies twice over when you are decorating a human body, though, because our bodies deserve to be treated with the utmost respect. Think twice and then think again about what sort of tatoo you choose.