For a start, who knows you best, your tutor and head of year, or you?
And secondly, who's future is it, your teachers or yours?
What i'm trying to get at is that it is you who should be making decisions about what you want to do in the future, and it is you that should judge what you can and cannot achieve. I let teachers and parents doubt my ability to do a particular subject, which put me off it, and it's something I really regret now.
If you are really interested in psychology then go for it! There is no point doing IT if it is not something you actually want to do. If you are in first year of college/6th form, maybe you can take up psychology as an AS next year. If you are in final year, maybe just look around at uni courses, see if there are any you would be interested in and could do with the qualification you are getting at the moment.
As for writing ability, I'm not sure, we all have our strong points and weaknesses. I am not great at writing either, and essays have been a bit of a problem, but I am slowely improving. And so long as you are interested in what ever you are writing about, I think that is the main thing.