Isaiah 61 is an amazing chapter. It is very meaningful to me, my wife, and many others who have gone through loss. However, I struggle with the words "instead of," as in "joy instead of mourning." Having lost a child, this is not our experience, nor do I expect it to be in this lifetime. I do have moments of joy, as well as waves of peace. But I don't expect a total replacement. And when I look at the original language, the literal word seems to be "for," which is translated "instead of." Maybe "instead of" is the proper translation, but "for" makes more sense to me, since my experience thus far is that God has given me moments of joy and peace are right there along with my grief and pain. So "for" means that God has given me these things as I focus and battle the emotional wound that I don't expect will be fully healed in this lifetime. I suspect I am reading this passage too literally, or that the passage is partially fulfilled in this lifetime and more broadly fulfilled in heaven... I want to ensure I am reading the bible from a healthy perspective, not letting my life experience or emotional state lead me to "dictate truth" or read too much into certain passages, or to cloud the message God wants to tell me. 
Yes, Isaiah 61 is an amazing part of the Bible. All of Isaiah is powerful -- no wonder it is the most quoted prophet in the New Testament. Grief -- even sustained grief -- seems to be part of the human condition, and you are wrestling with powerful emotions, especially as you strive to think theologically and biblically. This is to your credit. Let's look at the passage again (Isaiah 61:1-4)
1 The spirit of the Lord God is upon me,
    because the Lord has anointed me;
he has sent me to bring good news to the oppressed,
    to bind up the brokenhearted,
to proclaim liberty to the captives,
    and release to the prisoners;
to proclaim the year of the Lord’s favor,
    and the day of vengeance of our God;
    to comfort all who mourn;
to provide for those who mourn in Zion—
    to give them a garland instead of ashes,
the oil of gladness instead of mourning,
    the mantle of praise instead of a faint spirit.
They will be called oaks of righteousness,
    the planting of the Lord, to display his glory.
They shall build up the ancient ruins,
    they shall raise up the former devastations;
they shall repair the ruined cities,
    the devastations of many generations.
I don't want to read something in to the scriptures that isn't there, either. And I don’t want to over- (or under-) interpret the preposition. Yet I think you're squeezing the text a little too hard. Here's my take on v3.
"Oil of gladness instead of mourning" — think how public (and noisy) mourning could be in OT/NT times. Eventually things quiet down, though we may still mourn in our hearts (Ezekiel 24:18). Sadness tends to subside, gladness gradually filling the empty spots. (At least that is what we long to experience.) Yet it would be hard to argue that “95% gladness, only 5% mourning” would not be a valid instance of the principle here. Just as grief has stages, it also has degrees.
Consider "bind up the brokenhearted" (v.1). Perhaps the brokenhearted have healed significantly, yet still experience some degree of heartache privately. After all, each heart knows its own bitterness (Proverbs 14:10) — a principle that doesn’t seem to be erased in any except the most cheerful (or shallow?) believers. There are things that have broken our hearts, too. My wife and I are brokenhearted. Yet we still have the joy of Christ. How can this be?
The truth is, sadness and joy can exist in the same space. Consider Paul’s words about suffering in 2 Corinthians 1:3-11. So if our goal is to reach a place where there is no pain, we may never arrive. On the other side, after the last day has come, we have been judged, and have begun to enjoy our eternal reward, he will wipe the tears away (Isaiah 25:8). The God of all comfort will comfort us. Until then, we live in the overlap of the ages (the present world and the world to come), and frequently experience a mixture of strong emotions. And that's okay.