It sounds like you could have ADHD (maybe primarily inattentive) as the root cause of your mental problems. There are many here like you, myself included. You will find lots of useful advice on treatment here, but it's a long road and you can expect it to take years before you get this under control. Welcome on board though, and congratulations on taking the first steps toward overcoming your problems.
Nihilistic thinking is very common for people with mental problems, I suffer from it myself. I do advice you to not resort to street drugs to feel good temporarily. Real sustainable treatment can be found, although difficult and probably not perfect. If you suffer from ADHD, which is basically genetic dopamine (/norepinephrine) dysfunction, all your other problems may develop over time. I doubt you suffer from schizophrenia, because that is also mostly a genetic disorder; and also involves dopamine, but there are different deficits.
You seem to be in a very bad place right now and you need to start with some official treatment procedures. I suggest somehow getting on a low dose of an SSRI/SNRI. Find professional help now! If you want better results after you're been on one for a good time, you can discuss your options here.
Street drugs have probably never fixed anyone's life (long-term). If your doctor doesn't want to prescribe you anything, he's an idiot and you need to find another one.
Nihilistic thinking isn't the problem alright, it's a symptom, and a coping-strategy, to compensate for lack of pleasurable experience.
If you want be silly and think there's no hope, which isn't too uncommon, you'll only get lost in the short-term highs and horrid withdrawals that so much define street drug-"treatment". Before you know it, any amount of sanity you still have will be lost. Joining this forum means you're not already a lost cause. Don't come here and proclaim you want to destroy yourself - you don't need any help from us doing that.
I know Psychiatrists and Counsellors can be considered different here in Australia but I have no idea about the UK. If you told your doctor about your issue and he's not giving you immediate help or helping you asap, with appropriate support, try and see a new doctor who will.