Why do I find it so hard to open up to people about my feelings/emotions?

Since I was young I have found it difficult to open up to people, if I am upset I would rather keep it to myself and cry alone, I find showing people my emotions difficult and I haven't really ever been a huggable person, like with my mum sisters etc, I don't understand why, its almost like I am embarrassed to cry in front of my Mum.. at work if stuff is going on I would rather just keep it all bottled up as opening up is just so difficult for me.

I do cry and get emotional when I am alone, but I never want people to know.

its difficult because I have been through rough times, my best friend died when I was 18, that was difficult for me because people wanted me to open up to them and talk about it and I just cant.

I don't understand it... the reason I am asking now is because I had my horse put to sleep last night, and I chose to be there alone, I would rather grieve alone.. and I didn't even tell my mum but she found out and I cried in front of her which I found embarrassing and I look at other people and they are openly expressing their feelings.