To race or not to race

Please help, here is my dilema.

I injured my knee 3 weeks ago while running and have not been able to run without pain ever since. I have been swimming a little and biking a little with little resistance as to not put pressure on my knee.
Ran 1.5 miles about 5 days ago and couldn't continue due to knee pain. It was difficult to walk for the rest of the day.
I ran indoors yesterday with a knee brace and felt moderate pain in my knee at about mile two, however, continued to run another 2 miles. Knee was sore after, but could walk comfortably.

The last sprint race in my area, and only my third race ever, is on Sept 18. I don't know if I should go for it and hope my knee holds out or continue to take it easy until my knee heals completely and begin racing again next spring.

I've never had a sports injury before so I don't know what the experts say.

Any thoughts are appreciated. Thanks!!

Sheri