What happens with a tense opponent is that the guy is taking away the technique that you set out to do, so you should do a different technique. But, since you're just starting out, you don't know that many, or have them ingrained in you to flow easily from one to the other. That takes a long timne, so don't be discouraged. Eventually, the tense opponent becomes the easiest one to deal with. Trust your instructor.
The opponent to truly fear is the one who remains completely relaxed an on balance during the entire confrontation, even if it is just a friendly exchange of playacting blows, techniques and manipulations, or throws. That guy is VERY dangerous....