When you are angry, for example, the anger you feel contains the thought that (e.g.) some other person has done you an injury... and ought to be made to suffer in return. If your reason thinks differently... you are internally not just pulled in different directions; you are thinking ultimately contradictory thoughts, one through your anger [and] the other through your reason.
So writes John Cooper in 'Some Remarks on Aristotle's Moral Psychology' (Reason and Emotion, p.245, bold added). He continues:
For reason to persuade anger (in this particular case, or in general) is for it to get its own view of what is good to prevail, in the sense that this conception comes to be adopted by the nonrational part itself, as well. To cause this to happen, even intermittently, may require practice and training, of course, but in that process reason is not just exercising brute force, as one might in training an animal. it is, among other things, addressing one's anger, trying to direct its attention to features of the situation that will show one's anger... that it would be, or was, wrong to feel in that way. [If this process is successful...] The way you then feel about what has happened and about what should be done about it, is exactly also the way you think, for reasons, about it.
Further, on pp.246-7:
Now because Aristotle thinks that the truth about such matters -- matters of value in general -- is properly settled by reasoning, he thinks that this is the direction that the resolution of such contradictions should take. Your anger was caused not by what you rationally thought, nor by what you would rationally think if you gave reason a chance, but by (possibly obscure) causes lying in your recent or distant past experience. ... There is no good reason to think that your anger reflects the truth about the matters about which it is making claims...
However, it is important to bear in mind that the resolution can go in the other direction, both in individual cases an in general orientation. Faced with a conflict between how you feel about things in some respect and how you think about them, especially if this persists for a long time despite efforts on your part to adjust the way you feel, you will, faute de mieux, tend to adjust it in the other direction, by adopting in your rational thoughts a view about good and bad and right and wrong that conforms to the way you habitually feel: it is as if, under pressure from your stubborn desires, you decide on reflection that the correct way to determine what is good is simply to accept as authoritative how you feel. (Perhaps you then figure out some reasons to suit.) What forces us to take one of these two alternatives is the central fact that being rational creatures we cannot very readily or contentedly let the contradiction stand. We are moved, because we are rational, to resolve it in one direction or another.
But are there limits to the claims reason may make against our emotions? Cf. Moral Roots and Alienating Aspirations