Try to relax on the bike. Breathe deeply soften your grip on the bars and get the tension out of your shoulders. This is important because if you're holding the bars too tight you'll feel every bump, which will make you nervous.
Worse still, too fierce a grip will stop the suspension working properly, which will make the bike react badly to every bump. This will make you even more nervous, so you´ll tighten your grip still more... it's vicious circle.
So try to relax - and talk to yourself. Reassure yourself that you're in control, that you've seen what's happening around you and that you know what to do to stay safe.
Verbalizing helps, even if it feels a bit odd. Most importantly, try to understand why you crashed - and don't do it again.....
I crashed last year, broke some bones, and now I'm really nervous about riding again. Any advice?