Deep depression. It drains you of willpower, even the willpower to actually go out and physically kill yourself in some manner which terrifies you. Naturally if you have something in the house to OD on, the barrier would be far less.
Thats a pretty miserable answer, but the honest truth to a certain extent. If suicide is in practice quite difficult to accomplish, then you will have a natural barrier in place.
On a more positive note, if you can find something to distract yourself, then that will help. The more intellectually or physically challenging the better. Its extremely hard to hold much interest in anything if you are severely depressed, but if you do manage to find something strong enough to distract you from your suicidal thoughts that will certainly help.
On a philosophical note, you can always embrace your pain and vulnerability, reinterpret it in a positive light. If you believe it all has some purpose -- "suffering is good for the soul," "it will forge you into a better person," etc. -- then that may help soothe you a little. If you think about it carefully, you may find that your suicidal depression has made you into a better person : you have become more compassionate and have a greater appreciation for the pain of others, for example.