Q. What happens if I feel like I don’t belong in the Celestial Kingdom?
A. If my friend*, Johnny Depp, invited me to a red carpet event, I would feel out of place. Even if everyone knew I was there with him, and that I could be there–I would still be completely uncomfortable. Even if I tried to blend in with a designer dress and shellacked hair, I would feel like everyone knew that I was an imposter. I’m a housewife, for goodness sake! In a similar way, in order to feel like we belong in heaven, we have to be worthy and we have to have confidence in our presence there.
The first obstacle is that we aren’t worthy. We are mortal, God is immortal. We are flawed, He is perfect. We are dull, His glory would blind us. We are soiled with sin and hatred. He is clean and full of love. We truly don’t belong there. And if it was just up to us, we would never get there.
For this very reason, God sent His Son to bridge the chasm separating us from our Creator. Jesus Christ can take us to immortality, perfection and sinless glory. He can make us heavenly. It is up to us to accept His help. That’s it. To be humble in acknowledging our utter dependence on Him in taking us from our current state to feeling comfortable in His presence.
There is no sin so bad that He can’t atone for it. He can take the blackest soul and wash it clean–if we will let Him. He forgave Zeezrom after Zeezrom’s arguments resulted in a group of women and children being killed by fire. (Alma 14). Surely, you have done less than that. 
He forgave Paul after Paul assisted some others in stoning Stephen to death. Stephen was Christ’s own apostle. Surely, you have done less than that. He wants to help us and forgive us. That’s what He is all about. 
And while Christ’s cleansing sacrifice does fit us for heaven, we can still feel a complete lack of confidence in our presence there. Johnny might walk around so graciously, introducing me to everyone, but I would still be shrinking and sweating. There is a scripture that talks exactly about this issue, D&C 121:45-46
45. Let thy bowels also be full of charity towards all men, and to the household of faith, and let virtue garnish thy thoughts unceasingly; then shall thy confidence wax strong in the presence of God; and the doctrine of the priesthood shall distil upon thy soul as the dews from heaven.
46. The Holy Ghost shall be thy constant companion, and thy scepter an unchanging scepter of righteousness and truth; and thy dominion shall be an everlasting dominion, and without compulsory means it shall flow unto thee forever and ever.
This tells us how to acquire the confidence necessary for us to stand in the presence of the Lord (without shrinking or sweating).
1) Be charitable towards everyone. Be kind, watch for ways to help those around you. Pretend you are an angel, always on an errand of the Lord. Not only towards strangers, but members of your congregation at church (the household of faith).
2) Let virtue garnish your thoughts unceasingly. Think good, clean thoughts. Be happy and optimistic. Be grateful for things you have. Virtuous thoughts are in line with the commandments. Follow the teachings of the prophets, and look for ways to be obedient to the counsel that God has given us.
If you live this kind of loving, happy life, it makes you a loving person–a person like Christ. You start to feel more comfortable around the valiant. In my church meeting on Sunday, the speaker talked about people who don’t want to be thought of as too “churchy”. They like to stay up on the latest trends, and give their devotion half-heartedly on Sundays so that they aren’t seen as the archetypal Molly Mormon. They are cooler than that. But what usually happens when a person tries to keep one foot in the world and the other in the Kingdom of God, is that they get pulled toward the world and away from the church. He observed that even if “churchy” people seem overzealous, heaven is full of “churchy” people. And the more you get to know them and become like them, the more familiar heaven will be.
Even if you don’t feel like you would fit in there now, you are made of heavenly stuff. It is your eternal heritage and your eternal inheritance. The whole plan is designed to ensure that we return to our Father in Heaven–better, stronger, and smarter than we were when we left. Have confidence in that plan, too.
*I feel like Johnny Depp and I could be friends, given the chance.