Why do I stammer in front of people only?

Why do I stammer in front of people only? Hi guys, I am suffering from stammering. But I only stammer when I was talking to someone either physically or on a call. I don't stammer at all when I am alone talking with me in a mirror. Since I was a toddler I used to stammer, I started talking into a mirror and it helped me a lot. Now, I don't stammer at all while I was talking with myself, but when I talk with someone I start stammering and it really sucks when you're able to speak properly but you can't... Can someone please help me with their tips and tricks? Thanks in advance