We put a lot of focus on being able to remember things, beating ourselves up when we are forgetful. But your brain is wired to filter out unneccessary information, to declutter your mind, so that remembering what is important becomes easier. The ability to forget takes practice, but this gives your brain space to organise memories more easily.
How we forget
If you imagine your mind as a cupboard, with over-flowing drawers, piles of stuff shoved to the back, it is difficult to find anything.
Similarly, your brain can be overloaded with memories, past experiences, or with worries, ‘what if’s’ about the future. These memories get in the way of being able to forget what is not useful or helpful.
what we forget

You forget where you put the car keys if your mind is on something else at the time. If you can actively forget information you do not need, and just concentrate on where your are putting your keys, you will remember where you our them.
Mindfulness is about being able to forget what is not important right now.
when we forget
The mind is clever at filtering out experiences which are frightening, overwhelming, traumatising, in an effort to regulate our nervous system, to keep us calm.
If we cannot forget, scary experiences, living in a permanent state of fear, we become anxious. If our mind is cluttered and unable to filter this can leading to psychological issues such as post traumatic stress. Counselling can help you rebalance the fear/safety process in your mind and body.
why we forget
We forget to protect ourselves from sadness, fear, distress. Sometimes it’s harde to forget what someone did or said to us we found hurtful. being able to forget, these feelings can help to restore a sense peace with unpleasant experiences.
Having new pleasant experiences can replace negative thoughts, memories, so we forget negativity, and see things in a more positive light.
Learning to forget
So, going back to our ‘mental wardrobe’, we need to have space in our ‘mental wardrobe for memories to be safely stored. `Some we will need easy access to, such as remembering tasks, locations, where we have been, what we have done. Storing it on you phone does’t count!
Memories are made, the filtered, and filed for easy retrieval. We forget by letting irrelevant information go, not keeping it in our active memory, so we don’t keep it ‘just in case’. Memories can be happy or sad. We might like to hang on to happy memories, but want to forget feeling sad.

For example, you might imagine waves on a beach, washing away the sadness with each roll of the surf. This sad memory would gradually fade away, leaving space for more making new happier memories.
References
Fawcett, J. M., & Taylor, T. L. (2010). Directed forgetting shares mechanisms with attentional withdrawal but not with stop-signal inhibition. Memory & cognition, 38(6), 797–808. https://doi.org/10.3758/MC.38.6.797
Fawcett, J. M., & Taylor, T. L. (2008). Forgetting is effortful: evidence from reaction time probes in an item-method directed forgetting task. Memory & cognition, 36(6), 1168–1181. https://doi.org/10.3758/MC.36.6.1168