You didn't specify the nature of your injury. The main thing is to do things that don't aggravate or slow the healing process. Modify exercise to allow you to get the workout you need. I have had bad knees since a teenager, but I know my limits. Getting back in shape, I feel like I want to run, but I know I have a lot more weight to lose before I do that. Weather changes make my joints hurt. Yesterday a storm front made my left knee hurt so bad I did not want to do anything, but I was able to get in some tai chi exercises I chose tai chi for the very reason it is low impact. Today the pain was gone, and today I made up for yesterday. I have other aches and pains that limit me, some days more than others. The key is I try not to overdo it on the days I feel great, and I at least try to do something on the days I don't feel so good. Eating right and exercising have helped my joints not hurt as much, and increased my energy so that I want to exercise, instead of being a slug. Just be patient and don't overdo it.
The urge to rush back in is good for motivation, just take it easy, pay attention to the advice from your trainer so that you don't make the injury worse, and work out the parts that aren't injured. You can look into one of those hand-crank bicycles if you want your heart rate up. Depending on what's wrong, you could do pushups or modified pushups, or pull ups. There are stretching, yoga, tai chi, etc. moves that you can do that don't involve your legs.
It isn't the end of the world, even if it feels like it is. You can do this!
thanks for the support, I think last night I was freaking out because I have came so far and I am not ready to stop yet and this feels like a road block but really I think its just a turn in the road. My knee was feeling fine last night so I was pouting that I was told to sit still but this morning it hurts again and I didnt even do anything to it so hate to say it but my trainer was right!