“You seem forced” can be used to indicate what you’ve expressed
Your meaning may have been gotten better than you think. “You seem forced” employs a fairly common usage of “forced”, indicating strain even though there is not a physical force. Note that “you look forced” in your question’s title is a little different, having to do with the way that “seem” refers more comprehensively to apparent behavior and “look” refers more to physical appearance.
Another way to say this that would be specific to tone of voice, diction, etc. would be “you sound forced” which could be hedged a little as “you sound a bit forced” and would be helpfully followed by a bit of explanation or a question, such as “is everything OK?”¹ or “do you feel you have to talk to me?”
Expanding on the “look”/“seem” difference in “you don’t look yourself”
With “you don’t look yourself”, it seems like you are saying that something about the other person’s physical appearance is unusual compared with historical data or some assumed expectation (e.g. someone’s face is green or streaked with tears and you assume they are not always that way, even if you don’t know them).
If you were to say “you don’t seem yourself”, your comments would be understood to refer to someone’s overall presentation. This could certainly include physical appearance, but is more often used to refer to someone’s speech, body language, and other aspects of behavior. I believe this phrase would fit the situation you described.
Note for “you don’t seem yourself”: Similar to “you don’t look yourself”, you would be understood to either be making a comparison to ways you’ve seen this person act before or ways you assume they normally act. For example, you think that the person doesn’t normally put so much emphasis on their words or deliver them in a staccato rhythm.
The note from the end of §1 of this answer applies here as well. Since you’re guessing at something about another person, it probably wouldn’t be a bad idea to describe a little more what you mean by what you’re saying. I personally think it would be appropriate to ask a question aimed at learning more about how this person is feeling and/or what they think about the situation.
It's a little difficult to tell from your example what 'giving attention personally' means, but to cover both possibilities -
If the person you are replying to simply expressed interest in the matter at hand, the stock response usually is
Thank you for your interest in this matter.
If the person you are replying to has actually done something to correct/improve/interracted in some way with the matter at hand, a better response would be
Thank you for your attention to this matter.
as it implies an action of some sort from the person you are writing to.
Edit:
Your edit to your original post makes things somewhat clearer. A more fitting response for your situation would be along the lines of
Thank you for attending to this matter personally.
Best Answer
'Personal circumstances' does sound natural. Native speakers do use this phrase. Due to 'personal circumstances' I cannot attend. 'Personal reasons' is also good.
In the USA saying 'health reasons', 'health issues', 'family reasons', 'family matters', is common, because just about everyone has these things and they can mean just about anything, thus people won't think twice when you say it, and your privacy is protected. These phrases are used as excuses so often that the person you are telling this to might realize you are using these phrases as a way of saying 'I am not willing to tell you the real reason, and it is none of your business. If you find that you are saying 'personal reasons', or 'personal circumstances' too often (more than once) you can always, at least in American culture, resort to 'health issues' or 'family issues' and still retain your privacy.