questioning my pregnancy

Should I feel ashamed that tonight I feel like I no longer want to pregnant?
I'm currently 13weeks pregnant and was so worried up until now that I was going to have a miscarriage.
Right now I feel so alone and unsure if my pregnancy is right? 
I'm 19 and I have been with my boyfriend for over 4 years on and off. Although both of us were upset when we realised I was pregnant it grew on us that we were going to start our own little family and we were slightly excited.
Now all I fear is that I'm going to left on my own, I will have to do this as a single parent, I won't be able to be a good mother. 
I feel so isolated especially since I'm feeling very left out by friends, they no longer invite me anywhere or ask me to do things since I told them I was pregnant. 
How can I make these feelings go away? I was content with being pregnant and now not so much..