There are a few important points which I think can make this much easier for you to grasp.
The first is that even if Mary were omnipotent, omnipresent, and omniscient - if she were all-powerful, all-present, and all-knowing - this wouldn’t make her equal to Christ because there would be one thing she could never have - existence in and of herself. Christ, being God, simply exists. One might even say He is existence. He requires nothing else to exist, nothing else can exist without Him, and He, as Aquinas would say, exists by necessity. He has to exist.
None of this is true for Mary. She exists only because of Christ. She exists only by the virtue of having been created by Christ. She does not have any existence or substance of her own right, and apart from Christ willing her into existence at every moment, she would simply - pop - dissapear, ceasing to exist, just like the rest of us.
The confusion comes in because we tend to think of omnipresence, omnipotence, and omniscience as being qualities that define God or make Him unique, but this is not so. God is defined and made unique by His existence, not by His abilities. It is now what God can do that makes Him God, but what God is that makes Him God - namely, existence.
That being said, is Mary omnipotent, ominpresent, and omniscient? The answer is no, for one simple reason. Neither she, not any other creature, can hold any of these characteristics in relation to God. Mary does not have any sort of ultimate power over God. She certainly doesn’t know God completely; nobody can. Even for the most glorified soul in Heaven, there is always a part of God that cannot be known. To know God completely would be to be God - this is, consequently, exactly what Jesus is. He is the total self-knowledge of God, which is why He is called the Word. So Mary is not all-knowing, because there is that “part” of God that nobody but He can know. Whether she is omnipresent is another matter, because it depends largely on what a person means by it. She certainly isn’t present in the same way God is.
However, Mary is powerful and knowing up to the limit of what a creature can be. She is the most powerful and knowing of creatures. One might say she knows everything except God in His fullness, and that she is powerful over everything except God. The question is, is this just? Is this something God would do?
As Catholics, we believe that God did do this. He didn’t do it because He had to, of course, but because in His Love, He chose to. He followed perfectly His own command to honor thy mother. However, it is important to note that as Catholics we believe God does this for all those who come to Him through Jesus Christ, to greater and lesser degrees.
Through Christ, we are made “partakers of the Divine Nature,” according to 2 Peter 1:4. What does this mean? It means that God shares His nature with us. He shares with us His “abilities,” namely, His knowledge and power. It is God who can heal, but He shares this with us, and so the apostles could heal. It is God who has eternal life, but He shares this with us, and so we can have eternal life. It is God, in the Second Person of the Trinity - who is the Son of God, but He shares this with us so we can be children of God. It is God alone who Loves, and yet God shares His own Love with us so that we may Love. This is, in one sense, what it means to be in the body of Christ. We, being part of His body, share that which belongs to Him. Thus, He is the Mediator, and we all share mediatorship by virtue of being in Him. He is the one Mediator, but if we weren’t mediators in some sense, we wouldn’t be a part of His body. Mary shares the greatest degree of God’s Divine Nature, and so she shares Christ’s Mediatorship to the greatest degree, and so she is given the title “Mediatrix.” It is the very same principle by which all Christians are caled “priests.” We share in Christ’s priesthood.
It is in justification that we come to share in this Divine Nature, as the Holy Spirit is poured into our hearts (cf. Romans 5:5). As we, having been justified, Love in accordance with God’s will and do good works, we grow in this justification (cf. James 2:24) and so grow in how much we share the Divine Nature. Because Mary was, through the gift of God, conceived without Original Sin, at every moment in her life she lived in perfect accordance with God’s will. Thus, she grew to share in His Nature to the greatest degree a creature could, and this is the reason for all of the things that the Church holds about her, whether in her abilities, her titles, or anything else.
Hope this helps.