It's Stravas fault for using the misleading term "form". It doesn't really say much about your form, it's real meaning is when negative, then you have trained more than usual recently (especially in the last days) and if it's positive, then your recent training load was lower than before. It can be a useful indicator for someone who always trains hard and wants to take part in a race - they let the form come into the positives to be sure to be fresher than usually. But for every other circumstance you should just listen to your body, it knows when you are ready to perform.