I just went through the process of applying to clinical PhD programs (got interviews at 6 places, and got a fellowship from the one I chose to attend) so I'll offer my advice. Do you have a psychology advisor or know someone who is either currently in a clinical PhD program or has a clinical PhD who can proofread your statement? My advisor was very helpful in the process; he gave me great feedback and, after about 5 revisions, I had a great statement. So my first piece of advice: get as many people who are familiar with the demands of a PhD program to proofread your statement and give you honest advice and feedback.
Next, I would emphasize your research experience, no matter how research-oriented the program is. The reason I say this is that the PhD is a degree which, by nature, emphasizes research. Some programs may not have a very strong focus on research, but a PhD degree is historically a research-oriented degree. So, talk about your research experience and why you want to continue involving yourself in research (which I assume you want to do, since you are applying for grad school).
As for personal information, don't give out too much information. Don't say, for example, that you want to go into this field because your own personal therapist was very helpful to you. Write about your interests, emphasize that you want to continue learning, and you may want to mention what your career goals are. As for explaining one semester of bad grades, I would say something along the lines of "I have been a hard-working student and attained high marks throughout most of my undergraduate career. My transcripts indicate that my marks were uncharacteristically low during one semester, but I do not believe this semester is indicative of my overall drive, ambition, and potential to suceed and excell in this PhD program." I would definitely mention the poor marks, but I would not try to provide an excuse for them. Just remind the readers of the statement that you are a qualified applicant.
These are just my opinions that I gained from going through the process myself. Talk to as many people as you can who have gone through the process, and have as many people as you can read your statement. I'm not going to send you a copy of mine because I'm not comfortable with that, but feel free to send my yours and I can give you my input.
Hope this helped!