Hi Moey,
Welcome to the forum. Suicidal thoughts are just part and parcel with PTSD to be quite honest. I had them for about three years, some before I knew I had PTSD, and after diagnosis. The ironic thing is, is that suicidal thoughts are often linked to depression, so what do you do? You take anti-depressants! The funny part of that is, that most anti-depressants increase suicidal thoughts as a side effect. Go figure ha!
At a guess, your suicidal thoughts are a symptom of another major symptom, generally depression. The best way to decrease your depression, is through healthy eating, daily exercise and keeping your mind active, which means doing tasks, hobbies, working around the house, etc etc. If your sitting still, doing the usual PTSD reclusive actions, then your not helping these thoughts along.
I remember that just about every single time I got behind the wheel of my car, I would look at each pole as I drove, each substantial feature that if I drove my car into, my risk of death would be high, thus eliminating the pain that PTSD causes upon us. Whilst those thoughts where a daily part of my life for three odd years, I always told myself, and reminded myself, off what I had to live for, or even better, what I had to live for getting past PTSD. Family, friends, interests, looking forward to returning to some sort of normal life again, etc etc. Suicide is just one thought, and I am sure you can find many positive thoughts to outweigh it. If not, then you need to look harder. Suicide is a thought, and because you have these constant thoughts, it doesn't mean you have to action them.
If your on anti-depressants, go back to your doctor ASAP and speak with them about changing your medication to another type, and even again and again, until you find one that treats your depression, without the increased suicidal thoughts. If your strong enough, get rid of the things all together and replace with good daily exercise for an hour, healthy food, and healthy interests to keep your brain active from having time to think about suicide. If all else fails, and you really can't control yourself any longer, get back to your doctor urgently, tell them your suicidal, and go and have a rest in hospital for a while with some intense therapy to help you through the rough patches.
What you should never do, is consider yourself weak asking for help, even demanding it when you are getting very ill. Doctors will generally stop, or get you in urgently, if your situation is deteriorating rapidly. They are not mind readers, so if you ring and speak with their secretary, and they tell you that they have nothing for a month, then tell the secretary that your turning suicidal, and that will get your doctors attention very rapidly to help you out urgently.
If all else is failing for you at that time, get on here, and write whatever is coming into your mind at the time, and see if that makes the moment a little more bearable. It works, trust me. That will hopefully buy you enough time until your doctor responds to you.