Whether you have a mental illness, or you love someone who does, the "what-ifs" are something we need not give a voice to; I think at some point we have all done it. For someone with a mental illness it can go something like this: "What if I didn't have a mental illness, what would my life be like?" From there it can become "What could I have been?" or questioning how much better it could be. For a family member the questions can be "What if I had known sooner?" or "What if I had done things better?"
Remember, God created us the way we are supposed to be, each one of us . Read Psalm 139:13-16: http://www.biblegateway.com/passage/?search=Psalm%20139:13-16&version=NIV. He loves us just the way we are. He uses the very things we go through to form our characters and make us strong. He has a special love for those who have the hardest things to cope with. There is great reward for those who go through suffering in this life. Read the Beatitudes in Matthew 5: http://www.biblegateway.com/passage/?search=Matthew%205:1-10&version=NIV. God is also so forgiving. Many who cope with mental illness in their lives whether in themselves or others, carry a lot of guilt, much of it being false. Regardless of whether the guilt is valid, asking God for forgiveness can lift a huge burden off your shoulders.
If God loves us the way we are, I think it's time that we do the same for ourselves. Ask yourself this- how would you treat someone going through what you are going through? Don't you deserve to think of yourself the same way?