I'm not sure if my retainer fits, I can't tell! HELP!!!?

I haven't been wearing my retainer for weks now. I have finally decided to start wearing it. I made this decision last friday. It used to be really tight but now it is really comfy (sort of as in not so tight), now when I take it off it comes off easily where as in before It used to be hard to get it off because it was so tight. It hurts when I have been wearing it for ages. I can tell that my teeth have moved slightly backwards. There is a clear gap at the front down the bottom, when I push it up into place it goes a bit further up, but it goes down at the back where my jaws are & when i bite on it, it goes back up but the gap at the front reappears. I can tell that some of my front teeth have moved back a little. Does this mean that my retainer no longer fits me? If you have a retainer have you had this problem? I have an ortodontic appointment check up in about 3 weeks & a few days. Is there any chance that my teeth will move back if I keep wearing my retainer 24/7? I was supposed to wear it all the time apart from when I was eating. Please help!