I'm not sure if all this advice is going to be helpful, to be honest. "Relaxed" means so many different things, really. It might be more helpful to try and express what you mean by being "totally relaxed while training". Also, why do you think that's a good thing? Have your sensei and your seniors told you to "relax"? In that case, I would think that you'd do better asking them to explain by "relax", why that is good, and in what ways your current practice is not "relaxed".
If by "relaxed" you mean that training is stressful for you and you don't want it to be, time and more practice will cure some of that. Two caveats, though: 1)by definition, if there isn't any stress, that means there isn't any challenge, and that means there isn't a training effect; you might as well be on the couch watching American Idol, and 2)on the flip side, if your current training is highly stressful -- physically, emotionally or mentally -- then you probably shouldn't be doing it. No matter how head-casey people want to get about aikido training being the center of existence, it is a voluntary activity. It is not good for everyone. If it isn't good for you, you should simply walk away from it.
My sensei doesn't use the word "relax" so much, which I like -- I don't find it helpful to put someone in a situation of stress and action and then telling them to relax, which is generally defined as a passive thing. Instead, he talks about the specifics of whatever's wrong: not what you aren't doing, but what you ARE doing that should change. "You're stuck," is something he says to me all too often, and this may be what people mean much of the time with the constant exhortations to "relax". Being stuck means being stopped, becoming fixed, which means that before you can move or respond, you've got to start the motor up again -- it may be just a momentary pause, but it's there. It's a moment that a skilled opponent can take advantage of. In part it is mechanical (stance becoming fixed), in part it is mental (getting your mind stuck on what you want to do and not dealing with the situation as it is). And, having noted that I'm stuck, my sensei can always demonstrate how to avoid the stuck points in that situation. But it's very much something that's learned on the mat, not in an internet forum.