Why isn't my retainer fitting?

So i've had my retainer for a long time now and back when I first got it I wouldn't wear it very much (I only need to wear it when I sleep) so then when I went to the ortho they noticed and told me to wear it every night, so then I have been every night so last night I went to put my retainer in and it won't fit on my right back molar and I have to grid my teeth together just to keep it on! I have a ortho apointment tomorrow and the doctor will think that I havent worn it! it keeps sliding off and now my front gap which has been gone since i've been putting in my retainer is starting to come back because I can't wear it! So now he will think even more that I haven't worn it! what do I do?!