Sandhya, to combine Law and Human Behaviour for a proper education is a beautiful choice. We can go 2 ways about it. You haven't mentioned your Highest qualification but here you go:

1. Take up a free online Aptitude test. There are many websites that allow you to take free aptitude tests which help you understand where your skills lie the most. I completed my Bachelor's in Physiotherapy and did my Masters in Psychology for the interest in Psychology alone.
2. Or you can pick up Psychology in your Bachelor's and pursue Forensic Psychology or Criminal Psychology in your Master's degree. Even Military Psychology falls more or less in this line only. This way you would have traveled in both the boats by the end of the journey. After pursuing my Masters, I did my course in Sports Psychology because my passion was in Sports. And so if your passion lies in Law and Psychology, may be you can combine them both and make a career out of it.
3. Or you can study Law through regular mode and Psychology through Distance mode. UGC allows that now. So once you complete both the courses, you know which side to go because 3 years is a good time to decide. Good luck:)

Hey Sandhya, hope you'll be doing great!

Both fields are very good, you can go for any. Just give some time to yourself and freshens up your mind from stress. After that, think which field is more suitable to you. Take some points in your mind (like, which will help me more in future, in which you're more suitable, which is easy for you, where you can easily get in).

Hope my information is suitable for you. Best of luck to you for your future.