1. Shields don't last long as before. I am not sure but I am wondering whether this is caused because its not glyped yet. Btw I can't glyp the toon at lvl 22. Has this been changed.
Shield has been nerfed some. The glyph does not affect how much it absorbs btw.
2. I know we should use heals instead of flash heals but I still prefer flash heals cos of the casting time and not instances are redesigned with more AOE stuffs. I am alittle worried to use otherwise. However I tend to go OOM alot and need to drink between fights. This wasn't the case last time.
First, you're going OOM because you need to learn to heal without relying on your emergency heal. This is a cold hard truth, but the fact is, in Wrath we had very few mana problems and as a result we were allowed to get lazy and use fast expensive heals whenever we wanted. It's going to take some time to break out of that bad habit, but it's the future of healing regardless of class/spec, so if you're going to heal, you need to learn to get comfortable with your slower, more efficient spell. Two tips for that: 1.) start winding it up immediately after the pull, and always be winding it up any time you aren't doing anything else. If you get behind the damage, you're in for trouble. You can always stop the cast if you need to cast something else or if you don't need it to land. 2.) Learn to accept that not everyone is going to be topped off all the time. That one is hard for me, but it's something we just need to get used to. If DPS takes an isolated splash but they're fine, throw a renew out, or don't, but don't stop to heal them all the way up.
Second, you're going to have to stop and drink. Especially at low levels. Wrath spoiled us all. If the rest of your party doesn't want to wait for you to drink, then they don't have to wait. They can do the next pull with no healer while you finish drinking, and see how well that works out for them.
3. One of main reason I want to level a disc priest to learn to dps and heal at the same time but now healing is taking so much of my mana. Never really had this before when I was levelling my previous priest which is level 81 now.
I'd recommend you go back to the basics, get comfortable with healing as it is now first, then try working Smite in. It'll get easier at high levels anyway.
4. Errr normally deaths are very very usual when I heal but now its like an every instance affair. Whats happen to Disc Tree. Its not that good as before. Also as usual tanks are not the kind to allow you to drink. They just keep pulling and pulling no matter how many times I tell them to watch the healers mana. I tend to leave the group in the middle of the instance.
Deaths will happen, especially at the beginning of this expansion, and especially in pugs, because people don't understand that it's not just an AOE rushfest anymore. They will need to use cc. They will need to pull wisely. They will need to let the healer drink. If they refuse to do these things, don't think of laughing while you watch them die as mean, think of it as helping them learn.
Have i gone wrong or do I have to change alittle in my playstyle?
You have to change your playstyle. You are not alone in this; it's happening to healers of every class, especially those who learned to heal in Wrath. I'd recommend adjusting how you do things first before you make a final judgment on the spec. Disc is not now what it was in Wrath, so it may be that it's not for you anymore, but the only way to find out is to give it a fair shot first.