If it were me I would just put this particular project aside for now-being tense will make me knit a whole lot tighter, I know! I have a sock that needs frogging because I knit the heel so tight it looks like the sock (the heel part anyway) is for a baby (they're for me). I was nervous when I did the knitting as I am fairly new to sock knitting and I am sure that's why its so tight.
Maybe a little time away would help. I don't think I'd change size needles, but just try to relax~your first sock came out just right, so obviously you can do it. Stepping back and taking a break from it might be just what you need to get back into the groove, so to speak.
I'm sorry if that's not much help~I'm just thinking this is what I'd do in that situation. Even a day away from the project might be enough to catch your breath and relax enough to loosen the tension. Good luck!