"Faith as a grain of mustard seed" being enough to move mountains is a concept that is familiar to a reader of the Bible. I think it gets misconstrued. Or at least, I think I've misconstrued it in the past.
It easily can be interpreted as something like this: If you aren't [moving mountains/realizing what you hope for/accomplishing what you feel God calls you to do] then it's likely because you don't have enough faith. If your faith was as big as a mustard seed it would be enough to effectuate the desired results. So therefore, the remedy is to muster more faith that you have now.
But I think "enough faith" is an incorrect concept. Look at how the mustard seed sized faith is discussed in Luke 17.
"The apostles said to the Lord, 'Increase our faith!"
So, who is speaking here? The Lord's apostles. The ones who have been faithfully sacrificing much and following him around and trying to live his teachings for years. To do that would require faith in him. But they obviously think that they lack enough faith and need more.
"The Lord said, 'If you have faith as a grain of mustard seed, you will say to this sycamine tree, 'Be rooted up and be planted in the sea,' and it would obey you.'"
In other words, a tiny bit of faith (which is probably what the apostles would have admitted they had but which they were worried was not enough) is actually enough. It is enough to do any amazing thing that the Lord requires. Once you have determined to follow Christ and to do his will and are doing so, as those apostles were trying to do, you do not need to be anxious about quantities of faith, you don't need to beat yourself up about not having enough or plead for a greater quantity in order make happen what needs to be done, you simply need the faith in the Lord that you have. And even if it is only as big as a mustard seed, it is enough.
Hmm, faith....seed...Alma 32 anyone? There too, a seed is enough.
But what about the times when you exercise faith and what you feel called to effectuate doesn't happen???? Isn't that an indication that you didn't have enough faith?
Actually no. If that is the case then every Old Testament prophet who was called to cry repentance and did so in spite of derision and persecution and hardship and the people ignored him and actually became more entrenched in their wickedness (and there were a number of those) didn't have enough faith. Such an interpretation of those circumstances is rather ridiculous.
The simple fact that those prophets responded to the call and worked hand in had with the Lord and devoted their energy to the work was an indication that they, like those apostles in Luke 17, had "enough faith". In Luke 17, Jesus is saying, "even if it feels only as small as a mustard seed to you, with God it is enough."
In the verses that follow Jesus makes an analogy, comparing our service to God with that of a servant's consistent, unheralded service to his master, and closes it with,"So likewise ye, when ye shall have done all those things which are commanded you, say We are unprofitable servants we have [just] done that which was our duty to do."
I think Jesus is saying that he knows that his apostles feel that simply doing the faithful work that they've been called to do is not enough, that it's unprofitable, that because it is simply what God asked them to do and is expected of them and not wonderfully accompanied by praise and recognition or any remarkable or obvious manifestations of success, that their faith is not enough and that they are not doing enough. But Jesus' message is that they are trusting and following their master and serving as they are asked to to the best of their abilities and that is what God expects. And they should continue to do so. Even if the faith to do just what they are doing seems small to them,. With God as their master and with his grace it is enough.
If you must use a definition of increasing your faith perhaps it is best defined as as it is used in a rhetorical plea to God once said by Gordon B Hinckley: "And so, dear Father, increase our faith in Thee, and in Thy Beloved Son, in Thy great eternal work, in ourselves as Thy children, and in in our capacity to go and do according to Thy will and Thy precepts. "
Trust God and Jesus and their work, trust that you are God's child/servant and can do what he wants you to do regardless of the responses of others, and trust that his precepts will teach you how to do it in a godly way.
And though that may feel just about the size of a mustard seed to you, it is enough.