How do I stop being a doormat?

I’ve made various posts relating to my dilemma for the past couple days, but now I’ve finally come to the reason my life’s been shit. It’s because I’m a complete b****. I was sure my wallet was stolen but I let myself and others convince me that my wallet was left at home or in the car because I wasn’t aware enough to realize I had it until I got in class and someone stole it from me while I was laying at my desk asleep. I can’t believe I let this go on for 2 days in fact. By now my wallet is most likely in the trash somewhere with my driver’s license and my cash in someone else’s pocket. I’m 18 years old and I have the confidence of a toddler. I can’t believe I let this happen to me, but I’ve had enough of being a coward. Going to school tomorrow, I’ll file a report to see if anyone can at least point me in the direction to the asshole who stole it while I was taking a nap, but I need to make sure I never let another incident like this happen again. I need to stop being such a pussy, and start taking back what’s mine whenever someone tries to steal from me the second it gets taken.