How can I practice my body language alone, in the comfort of

A lot of nervous tics need to be treated from within, they will disappear if you are actually more confident.

Train your posture. There's lots of exercises from balancing books on your head while walking around, to standing up straight against a wall and focusing on the shape your body takes like that etc. It's been a long time for me so I wouldn't know what the most effective is, google around a little, but I had mediocre posture as a teenager and corrected it to the point where my posture is one of the most common things for me to get complimented on.

And this is a protip I happen to have learned from my father: focus on a point, look straight ahead with your head held high. Then shuffle your feet one way then the other, so you are turning sideways, but turn your entire body while keeping your head still. Basically keep making exaggeratedly intense eye contact with this one spot even though your body is facing different directions. This is a very effective exercise for people who have trouble looking others in the eye, and making too little or too fleeting eye contact is one of the biggest things that makes people look insecure/unreliable etc.

First of all, it's important to find out for yourself, what those "reactions" you are talking about are.

For example, I used to scratch my nose when I was nervous (often considered a sign of lying btw.). Took me some time to notice it by myself. There are probably a lot of things you don't know about, because you never really paid attention to it.

Look up, what signs of confident body language are and practice those (don't have the time to explain every single thing. There are a lot of helpfull videos about body language and self-confidence on youtube).

Also, the next few times you are in a situation you are usually nervous in, try to focus on your body language and be aware of how you act. Rehearse your behavior in those situations.

And don't try to avoid this kind of situation, embrace it. You will only get better trough practice!

Good Luck!