The short answer is that God's power is not at our disposal. I think the answer becomes a lot more clear if we consider a few things.

1. God is infinite in understanding, power, wisdom, and knowledge
2. We are finite in all aspects of our lives
3. God is the creator and sustainer of the universe and we are one of his creations
4. God is sovereign, meaning that he does whatever he wants
5. We must always remember that God loves us

Considering these, imagine how things would really be if God's power were at our disposal. What if we who are limited in our understanding, knowledge, and especially wisdom could wield the infinite power of God as we desired. The human race would have ended a long time ago. We would have destroyed each other with such power. We cannot handle atomic power in a completely constructive way so how would we be able to handle the power of God, which is far beyond any nuclear power.

Humans for one thing could not be given the true and full power of God because we would not be able to handle it. Remember that we are finite. Besides, the Bible says that Jesus Christ was given all power in heaven and in earth, that is the entire universe (Matthew 28:18). There is nothing in the Bible that suggests that the power of God is at our disposal as if we could use it as we saw fit. The power of God is available to us according to HIS will, not ours. Consider the following scripture for example.

"Now this is the confidence that we have in Him, that if we ask anything according to His will, He hears us. And if we know that He hears us, whatever we ask, we know that we have the petitions that we have asked of Him." (1 John 5:14-15)

Now compare with the following scripture.

"So Jesus answered and said to them, “Have faith in God. For assuredly, I say to you, whoever says to this mountain, ‘Be removed and be cast into the sea,’ and does not doubt in his heart, but believes that those things he says will be done, he will have whatever he says. Therefore I say to you, whatever things you ask when you pray, believe that you receive them, and you will have them." (Mark 11:22-24)

The mountains represent things that are deemed impossible (this was a common metaphor in rabbinic teachings). These are the things that we surely could not do by ourselves. However, notice what Jesus says first. He said to have faith in God. It is not that we can control or use the power of God at our disposal. The former verse shows us that our prayers must be consistent with the will of God in order for us to receive what we pray for. In other words, if God doesn't want it then it is not going to happen, unless of course we move into disobedience and God permits the thing to happen. It is my faith (confidence, trust) in God, which is trust or reliance upon God that I am able to move mountains, i.e., do what seems impossible. I can't use the power of God as I would use electrical or mechanical power at my discretion. God uses his own power as he sees fit.

Also keep in mind what Jesus continues to say in Mark 11:25-26.

“And whenever you stand praying, if you have anything against anyone, forgive him, that your Father in heaven may also forgive you your trespasses. But if you do not forgive, neither will your Father in heaven forgive your trespasses.”

We should forgive when we pray. Our prayers cannot be answered if we hold unforgiveness in our hearts among other things.

Our task is not to use the power of God, but to obey God and fulfill his purpose for our lives. This means that we have to do all that we can to remain in His will. Only then will we be able to pray with confidence and know that God will do what we ask. Why? It's not because we have the power of God at our disposal so to speak. It is because we are part of what God is doing in the first place and that means that what we do will therefore prosper.

So God's power is not at our discretion as we don't have the wisdom to use it properly. We are ill equipped to wield the power of God, that is the power the created all things and sustains all things, at our discretion. The only thing that we can do is hang on as God wields his power around in the earth to carry out his will. This might give an illusion that we are using his power, but in fact we are merely walking in obedience. Solomon said it very well.

"Let us hear the conclusion of the whole matter: Fear God and keep His commandments, For this is man’s all." (Ecclesiastes 12:13)

We are always at the mercy of God. However, God is merciful so we are always in good hands so to speak. God's wisdom transcends ours in immeasurable ways and there is no way that we could control him with our limited understanding, knowledge, and wisdom. We can only do with what He has revealed to us about himself and his creation. We can only really get on board so to speak with what He is doing.

Sure I can say "Mountain be removed" and a slew of other confessions. But the truth of the matter is whether or not God has purposed that mountain to be moved and whether we should move it.

This doesn't mean that our prayers are useless because the Bible admonishes us to pray. The Bible encourages us to pray and the Bible teaches us to pray. Revelation illustrates prayers as real substance before God. We don't understand all that happens when we pray, but we do know that the a fervent prayer accomplishes a great deal (See James 5:16). However, we can't make anything happen just because we pray. God is sovereign and He answers our prayers. He decides whether to answer or not. So God's power is not at our disposal to use at our discretion. That couldn't be especially since the Bible doesn't say that it is. If anything, we are at the disposal of God so to speak. We do what He wants and not the other way around.