Yeah having someone tell you to relax doesn't help at all and (for me) usually has the opposite effect. I've noticed that not thinking or putting too much thought into my singing has actually helped me do better because I'm not trying to force anything to happen, it just flows naturally. 

Yeah, that sucks cause we never understand mentally that kind of instruction, our brain only does things like positively and in one way, and it is not so good at not doing stuff or following a negative instruction; I mean right now if you think "Stop thinking" you will only keep thinking about not thinking and everything you can imagine or think of will be immediatly translated into mental imagery so it's impossible not to think about things, and in my little experience relaxation only comes naturally when you focus all your energy in only one thing, when singing I only get fully focused when I pay attention to the lyrics and the meaning behind, and all of a sudden I can stop thinking in anything special and my mind just flows, but in the moment I realize I'm thinking I fall to the overthinking trap again, it can be frustrating man.