Don't get worried, every asana comes naturally when you put much work to it.
That's possible your muscles could be a little bit too weak to handle your body, so let's try some core exercises. I posted today about it on my blog -> http://planetofyoga.com/en/bakasana-core-training/ here you will find a film with one of the trainings I do to hold bakasana or parsva bakasana which is side crow ofcourse.
Try to focus on your arms, back, belly and chest. This will support your posture better.
Try also to find how to get into this pose, find some preparation exercises in the internet. I will post some video with parsva bakasana also so you could follow my blog and soon there will be some tips.
I hope I helped you somehow, please let me know if you need some more help and feel welcome to write me on my site!
Have a nice day/evening!