I've found that writing down my fears and doubts before a tough run helps me confront them head-on. It's not about ignoring them, but acknowledging they're there and then telling myself to shut up and get moving. Also, I've started using a 'everse psychology' trick on myself - when I feel like giving up, I tell myself I'm not even tired yet and that I can do way more. It's weird, but it works for me.