How Can I Fix my Retainer Without Seeing my Orthodontist? (photo)

Looks like your options are limited. There is no way to repair this brackage proprerly. You can smooth down back part and hope for the best. I would not recommend that though, because back teeth can supererupt and mess up your bite. 
Please consider getting a new retainer. Any dentist can provide it for you. You do not need to see an orthodontist.

Unfortunately, you will need to see your orthodontist and have a new set made. Using glue will not fix it properly, it will create a space where the glue is connecting the two pieces resulting in it not fitting all together. I would recommend seeing your orthodontist as soon as possible so your teeth don't shift even more.

I don't think you will be able to fix that retainer so far as gluing it back together.  I am an orthodontist and would suggest that if you just can't get in to get another one made or can't afford it right now you probably could get by for a while just wearing it (the larger piece)  like it is if you have been wearing it at night time only.   You may want to smooth out the rough edges where it is broken off with a nail file or emery board  so it is smoother on the tongue.  There may be slight extrusion of the teeth where the piece is missing  and they might feel a little high in the morning but it will probably go away OK.  Try to get a new one as soon as you can.

Unfortunately not. You will have to see an orthodontist to have a new retainer made. You didn't say how long this one has lasted. You might want to discuss your grinding when you go in and discuss your options.

Vijay Bhagia, Space Center Orthodontics, Houston TX