I do believe there is a link between mental toughness and physical toughness. I think there are a few things you can do on your own that will translate into the dojo too. I don't know what your personal training is like, but I would start doing some sort of exercise at home. I would recomend saburi with a very large bokuto and sumo shiko (leg stomps). If you're not familiar with sumo shiko you should be able to find some video online. It's a lot harder then it looks. Either way I would work out in such a way that challenges you. If you've reached 500 cuts with a sword (or 100, or 50 what ever your limit is) push yourself to get 5 more cuts in. If you drop the sword oh well no one is around to watch. If someone does see you drop it from fatigue let them know you were working out until failure, what can they say? You're a hard a** who works til you drop. Nothing to be embarrassed about there (even if you only did 503 cuts... they don't know that) Unless you have some sort of medical problem you probably won't die (if you do die sorry for the bad advice
). Same thing with what ever you do, one extra push up per set, or ten extra sumo shiko, what ever. This will give you a number of small victories to build on. For my shodan test in Daito-ryu we had to do 100 ukemi in a row with someone throwing us. we worked our way up to it gradually. At our test when we did it we all felt very good.
Cut out sweets and bad foods for you. Eat a lot of protein and veggies and whole grains (unless you're on a special diet). You are what you eat, so if you eat cream puffs you'll be a cream puff (I'l admit I'm a cream puff sometimes... okay most of the time... okay I'm more of a cookie or donut but whatever...I eat a lot of good food too).
As silly as this sounds kiai-ing into a pillow can help, too. Or you can go into the woods and do it. either way kiai is great for building yourself up.
I don't know how you carry yourself either, but if you slouch or have bad posture try sitting and standing straighter. This will project an air of confidence even if you don't have any. Getting into the less physical side, sometimes (most times for me anyway) if you act like you should be there or you know what your doing people will take you more seriously. I don't mean become a paper tiger, or a know it all jerk with no clue. The first time I met Kondo sensei we had to do all of the first 30 kata in front of him, While I was crapping myself on the inside because we really didn't know the kata very well, we did the best we could and went 100% with each kata. I think if one of us missed the technique we may have actually knocked each other out. It didn't matter that we didn't know what we were doing, I'd like to think the reason sensei took interest in us was because we were willing to go 100%.
Work out with the largest most experienced person in the dojo. Look at it this way, if you get thrown by the biggest dude there, no one would expect anything different. Then when you throw the big dude take some pride in the fact you threw the big dude. If you work out with the newbies and small people and get tossed around what is that doing for your ego? I've found I'm more likely to get hurt by a newbie anyway... stay with the more experienced people because they're less likely to hurt you (or they should be anyway).
Also talk to your sensei and let him/her know your issues, maybe in private. You don't have to word it exactly like you did here. Tell them you need help with gradual challenges in your training. Maybe let them know the class is moving too fast or slow for you.
Keiko should be tanren, forging the body. Burn off the dross. If you have a lot of dross it will take longer to get to the pure iron or gold or whatever you're made of (cream puffs? or maybe Oreos like me?), but keep working at it.
Your head is in the right direction keep showing up. You've found your weaknesses now go to the dojo and get rid of them.
Kashima Shinden Jikishinkage-ryu kenjutsu