The best advice I can give you would be to take a break. Give your veins time to heal and your body time to reset it's tolerance. I understand that you want to be high, I want to be high too (I'm a junkie though, not a tweaker) but you said you aren't even getting high for more than thirty minutes anyway. You're basically destroying your veins and for nothing. Even when he misses you should still feel something, the drugs
are still in your body after all. I used to shoot meth
a lot and it's the same as heroin
in this regard because obviously you want to hit the vein and get that rush but if you miss you will still feel it, it just takes a little bit longer and there's no rush. The fact that you aren't feeling anything means you really need to lower your tolerance and injecting is having the exact opposite effect.
You really need to learn how to hit yourself if you're not going to stop. You said your dealer has injected you 6 times and 4 of them he missed? That's a terrible track record. For every good shot he gives you he gives you two bad ones. And those bad shots aren't only not getting you high, they're also making it so you can't get high in the future with those veins and could potentially lead to an nasty abscess.
You should get a dishcloth, get it wet, put it in the microwave for like 15 seconds, then hold it on the area where he missed and massage it. When the rag gets cold heat it up again and repeat. The more you do this the quicker your veins will heal.
The best place to inject would be inside the elbow. That's why when you get blood drawn or an IV put in they use that area. But there's tons of veins along your arm. Use a tourniquet to see them better. Make sure your warm (veins hide when it's cold), do some push ups or jumping jacks, and make sure you're well hydrated. Make sure you register each time. Even after you've registered about half way through register again to make sure you're still in the vein. It shouldn't burn or sting. I remember it being really hard to miss an IV meth shot because if I missed even a tiny bit it would burn and sting and turn red instantly whereas heroin takes a little bit of time. So if you're injecting and think "ouch, that kinda hurts" re-register and I bet you'll find you're not in the vein.
Always use new rigs, this will help reduce damage to your veins. Always release the tourniquet before injecting. Be kind to your veins, once they're gone you're stuck using your feet (which HURT) or risky areas. If you inject in one place don't use that vein again for minimum 3 days, you need to give it time to heal. There's lots of good information on injecting in the heroin forum as well.