This is approximately my response:
Hopeless self-criticism is only pride in different clothing. Someone who is poor in spirit accepts who they are. Getting to this place, however, for some Protestant converts often is experienced as a kind of see-saw. This is because most Protestants have inbred in them a theology that God loves and accepts them when they are good–i.e. first you repent (change) then God loves and accepts you.
It takes a long time to reorient oneself in the Church. God’s love and acceptance never, ever, ever, ever changes because God never changes. We only hurt ourselves and create obstacles in ourselves when we misbehave. For many people, sin functions a lot like a stress induced headache. The more you think about how to fix the headache, the worse the headache gets. Actually, asthma also functions that way in some people (like me for example). For me, the trick was to teach myself not to become anxious when I felt an asthma attack coming on. I had to accept that this undesired phenomenon was happening to me and peacefully put myself in a situation away from what seemed to be causing the irritation and where I could calmly focus on relaxing and breathing regularly until the worst of the asthma attack passed.
In the same way, some sins sneak up on us and take us by surprise. Sometimes we are well into “sinning” before we realize what we are doing. When that happens, when we realize that we are in the middle of sinning (in the way we are thinking or talking or acting), then we must calmly and peacefully withdraw as much as we can from the situation (even if the withdraw is merely to stop talking or to busy ourselves with other things). We do not need to become angry with ourselves–it doesn’t help. We need merely to recognize that we are poor, sick, and spiritually lame. We can pray, Lord have mercy.
However, for some, Protestant training has taught them that sin is a choice, in the same way that making a left or a right turn is a choice. But sin is very seldom that way. Those who do not want to sin are most often tricked into it (you might say) by mechanisms that are mostly subconscious. Learning to recognize those mechanisms in ourselves is a life-long process.
And while we are learning, we must never despair of the love of God. In fact, it is God Himself, the Holy Spirit within us, that is enabling us to see our sin. Were it not for the Love of God and the Grace of God already at work in us, we wouldn’t even notice that we sin. So that you notice is evidence of God’s love for you.
Humility, or poverty of spirit, requires that we hold these two things in our heart at the same time: on the one hand, God’s unchanging and unfailing love and acceptance; and on the other hand, our chronic sickness which will not be fully healed until the age to come. And this sickness is sin.
Spiritual disciplines, loosely categorized under the three headings of Prayer, Fasting and Almsgiving, help us learn to manage this sickness and to begin to see its root in ourselves. This helps us recognize the development or evolution of sin within ourselves while it is still just a thought and learn to deal with it there, in our minds and hearts, before it becomes a determined thought effecting our feelings, words and actions. But just like any disciplines that can bear good fruit in our lives, spiritual disciplines require practice over time and bear fruit slowly.
So, to return you your original question, “how can I become poor in spirit,” you become poor in spirit by owning your poverty without despairing of God’s love and acceptance. But this will require time and a certain amount of retraining yourself not to think of your sins merely as failures of choice (failures you could remedy if you worked harder); but rather to think of your sins as manifestations of a deeper sickness that you are just beginning to learn how to manage.
People who get type 1 diabetes as teenagers have to learn to pay attention to their bodies. They cannot get mad at themselves for feeling dizzy. They have to learn to recognize imbalance in their system and what to do about it before it becomes dizziness. Similarly, through prayer, confession, and other spiritual disciplines, we begin to recognize in our souls the beginning of sin and through experience learn what to do to “nip it in the bud.” The fruit of this growing understanding and inner work of “nipping in the bud” (aka repentance) is the beatitudes, or the fruit of the Spirit.