If it sounds strained then it's probably because you are tensing or trying too hard. You should discuss this issue with your teacher but some problems are resolved with time and practice which may be the case for you.
Make sure you relax before you practice - there are lots of tips and advice for singers at our parent site
http://www.vocalist.org.ukand you can post singing related queries at
http://www.singing-teachers.co.ukcommunity discussion forums.
Hope this helps
KrazyKat Admin