Hi Sophie, I'm 28 and have one of these on at the minute. I've had it since January and have it for another 4 months.
For the first three or four days I really couldn't talk properly. I felt like I sounded like a deaf person trying to speak. However everyone else assures me they could understand me just fine! After a week I felt less self conscious and after 2 weeks I didn't notice them anymore.
Its the same for eating and drinking too. For the first week I felt like everything I put in my mouth went straight into my retainer and I was spitting and drooling. Now I can eat and drink just fine, and just take it out after to rinse it. The more you practise the better you will get. I ate on my own to start with in case anything happened but that feeling soon passed.
For speaking, the best advice I can give you is to buy a magazine and read it to yourself out loud. Your mouth and tongue will soon figure out how to rearrange themselves around the new hardwear in your mouth!
Lastly please wear it exactly as much as your orthodontist tells you too. Depending on the problems you had fixed, it might be necessary to keep it on all the time, even if some people only have to wear theirs at night. Just remember how quickly your teeth moved every time you had your brace adjusted - they started hurting pretty quickly didn't they? Your teeth can move BACK into their old position just as quickly if you don't keep the brace on.
Stick with it and you'll be gorgeous :)