We are commanded to love Hashem with all our heart, all our soul and all our might. (First paragraph of the Shma.) Clearly, we need to serve Hashem with all parts of ourselves -- our bodies, our minds, and our emotions.
It is not easy to change our emotions, just as it not easy to refrain from sins. But the ideal is that our head rules over our heart. We have to try.
Let's say I keep getting angry although I know I should not get angry ("Let my soul be silent to those who curse me..." from the post-Amidah prayer). Orchot Tzaddikim says if I have an angry temperment, I should adopt the opposite mentality, never raising our voice and such even when it may be appropriate, until I have accustomed myself to avoiding anger. To be honest, I am trying to implement this in my own life!
Another example: let's say you know it is a commandment to love Hashem (one of the six constant mitzvot.) Then take a few minutes to meditate on G-d's greatness, as recommended in Chabad chassidut, which says that this will lead to a fiery love of G-d.
Similarly, with forgiveness, this is not easy but it is not all or nothing, and with effort we can achieve at least partial forgiveness. See this essay on the Lubavitcher Rebbe's discussion of different levels of forgiveness.
If we have difficulty achieving these things, we have to keep trying, and as Rebbe Nachman explains, we should pray in our own words, because prayer always helps (see Likutei Eitzot, Hitbodedut, Tefilah). Both chassidic and mussar literature contains numerous suggestions on controlling our emotions and cultivating positive emotions such as love and fear of G-d.