"My heart is steadfast, O God, my heart is steadfast" (Psalm 57:7)
"He will not be afraid of evil tidings; His heart is steadfast, trusting in the Lord. His heart is established" (Psalm 112:7-8)
There are peoples and peoples. Somebody, who is driven by feeling, is as the wind. A moment he is there, another moment he is another place. He does not even know where the wind will carry him. This kind of people can cause serious damage to the others, because they are not committed. Well, from their point of view, they might be committed, but they have no control where the wind will carry them next time. So, a moment they love, another moment they love no more. Partially, it does not even depend on them. They are slaves of their changing feelings.
Despite the former category of people is the most common, there are people who enthrone the Lord in His place. Their heart is an understanding heart, because it is governed by the brain. Their feelings spread out from the will to love and are steadfast. The will continuously tells the heart to love and how to love, and directs the wind of the feelings.
I think I belong to this category. How do I know? Well, if I look at my previous experiences, my love did not turn into hate. I am also consistent with my feelings. I see that love does not decrease. There must be the intervention of the mind to say, "That person does not deserve this; they harmed you so and so; love should be turned into pity".
Just for the sake of completeness, there is another big category of people who enthrone their body over their soul. Those just look to fulfill the pleasures of the senses. Of course, they are very mutable and untrustworthy.
It all depends on what you enthrone in your inner seat (i.e. which part of you will have the command). It can be either the soul, the spirit, or the body. This determines the kind of person you will be. What do you think?