Ah, the ol' "tense up" scenario. Well, I'm no old pro, but I've been dealing with a similar issue as you. I've just broken double digits for my hours, and haven't done my solo yet...so take everything I say with a grain of salt (although, going through this right now may add to my credibility
).
While I don't have this problem on takeoff, I do tend to tense up a bit on roundout and flare while landing...so I'll share my tips for relaxing. I'm confident your problems are similar to mine, so I hope this will help.
When I'm landing, I have been tensing up just over the threshold, and then overcorrecting everything. Sound familiar? Sounds like you are doing the same on takeoff. The problem is excacerbated by fixating on something...even if it is just the runway right in front of the cowling.
To overcome this, what has been working for me is literally sitting back in the seat (a more relaxed, comfortable, nonchalant postion), loosening my death grip on the yoke, and, the most important part, looking further down the runway to get the "big picture" instead of just over the cowling.
I'll bet my bottom dollar that you are looking right over the cowling and trying to follow that centerline. Think back when you were learning to drive, and you didn't have the "sight picture" for that yet either. I vividly remember driving my dad's truck on the highway with my learner's permit and trying to keep the hood at the same point along the line on the side of the road. Can you imagine driving like that now? It's very, very difficult, and I bet you are doing something similar now (I know I have been during landings).
So my advice is, sit back, relax (force yourself), and look down the runway like you are driving on the freeway (it's almost the same speed near rotation). Act like your in a 70's towncar or some other car you can really "lean" in and just lean back and relax
. I'm sure this will go away as you get comfortable (experienced).
Good luck...and keep us updated.