Re: Mental Toughness
Learn to push yourself physically, and the mental stuff will follow. Set small goals for yourself. If you normally walk, force yourself to jog a small part of the distance, and every day push yourself to jog a little bit further. Eventually you'll see that you can do it despite yourself.
At the dojo, wait until class is over and you're already exhausted and find a trusted aikidoka that *will* push you to your limit. Always keep in mind that you have the ultimate control over your situation. You can ask your partner to slow down or stop any time you need to. That's the beauty of the dojo: it's a laboratory where we can test our own limitations with the safety net of being able to stop the experiment at any time. Also, don't indulge yourself in negative thoughts. If you are working on irimi-nage, don't think to yourself "Oh I always mess this up!" as you're about to do the technique. Allow your body to do what it already knows how.
Finally, and this is probably the toughest advice to put into action, is to put trust in your fellow aikidoka and your sensei. Trust that they are not there to make fun of you and if you make a mistake or get exhausted and can't push yourself any further that they are not going to judge you for it. In my experience Aikido people are often sincere, good-hearted people that will help you if you let them.
You asked for specifics, and this is exactly what I am continually working on.