I wish I had a nickel for every time Yahweh utters the word "forever" in the OT in making some promise the particulars of which turn out to be very much temporary. Either Yahweh can't see the future, or Yahweh frequently changes his mind, based on humans acts. It would seem that Yahweh is neither omniscient nor unchanging.
First, "forever" appears 258 times in the NIV translation of the Old Testament, so if you did have a nickel for every time Yahweh utters it in a particular context, you could have a nice lunch but not a bottle of wine with it.
More to the point, the Old Testament was not written by philosophers. And more importantly, it was not written to philosophers. The idea of God as unchanging is not only misunderstood by a lot of people (including a lot of Catholics, despite their professing a faith which holds that God is unchanging), it is regarded as a positive assault on their faith.
For practical purposes, then, we can get away with saying formally untrue things like "My sins sadden God" or "God responded to my prayer."
Getting back to the Old Testament, it was written from the perspective of Israel, not of God or some disinterested third party. From the perspective of Israel, when Israel flourishes God is rewarding His people; when Israel is led away captive, God is turning His back on them. When Israel is faithful, God seems happy; when Israel is faithless, God seems angry.
Does that mean Israel's faithfulness makes God happy, or her faithlessness makes Him unhappy? No. God's love is unchanging. It is Israel that changes, and when you change your experience of God's unchanging love changes.
The analogy I like is from St. John Fisher. Sunlight is warm and cheery when you're healthy, harsh and bitter when you're sick. Imagine waking up to the sun on your face after a good night's sleep, then to the same sun after a hard night's drinking.
I'm not arguing here that "God and His love are unchanging" is true. I only mean to show that pointing to examples in the Old Testament of the LORD growing angry or changing His mind do not prove it isn't true.