I was not an only child but the youngest of nine, and I had the same problem trying to control my desire to please everyone . As long as you are willing to relinquish control when you see yourself using them as an excuse to not pursue your own vocation, you will not become a stalling to your own calling. It will never be easy to tell them the truth about what you feel, nor to leave them, nor to try and change them just a little bit more, before you move on. Ironically, they change for the better when you show them the example and stop trying to change them before you change yourself, usually. Dont' make them a stalling for your calling.
What I suggest is that you love them according to the Gospel more and more and less and less according to your fears and need for security and control. Remember, your past affects you more than you think. Seperation of your parents at a young age, as an only child, leaves you feeling responsible to fix it up, make it all better, as you struggle to not drown in isolation and loneliness, feeling orphaned, split up inside, like someone holding on to the debris of the Titanic after it sank. You want to swim out to your Mom and Dad and make them get into the lifeboat, but you have to get in first, to use analogy. The lifeboat is God's Will, grace reaching out to floaters in life's ocean. You will always be tempted to make sure your parents are o.k. before you do anything, that can become a form of tyranny for yourself and you will get mad at yourself for not being able to make them happy, giving yourself a mission impossible, a cross Jesus never gave you.
What I suggest practically is help your Mom connect as much as possible with her siblings and your Dad with his and with you and especially with Jesus. Jesus firstTell your parents about how you feel Jesus calling you to be all for Him, and your fear of leaving them, honestly, openly, with confidence. Jesus will give you the strength because you are in love with Him. They will see that, and, if they love you, I am sure they do, they will try to understand you, and yes, be afraid of losing you and yet they will see that you are serious about this all at the same time. They don't need to agree with you, only respect your choice. You see, they cannot control you nor impose on you what they think would make you happy anymore than you can on them. It works both ways.
You are living the Gospel passages about loving God more than family. Remember Jesus' words which sound harsh or too demanding- Whoever leaves Father and Mother, Brother and Sister...for my sake or- Whoever puts his hand to the plow and looks back is not worthy of the Kingdom of God, Let the dead bury their dead... or especially-Whoever does not hate Father and Mother, Brother or Sister for my sake, is not worthy of Me...meaning whoever loves their attachment to them or loves them more than Jesus without whom they would not exist. We should desire Jesus' approval, praise and esteem even more than that of our parents. He will love after them, they must choose Him. They must get in the lifeboat-if you wait for them or make them a condition for your Yes to the Lord, you might miss the boat.
Mother Teresa's own mother spent a whole day in her room, when Mother Teresa told her, at eighteen years old, I want to become a nun...then the Mother came courageously out of her room and said: Go! Take Jesus by the nand and walk with Him-do not look back, if you look back, you will go back or you will come back. That is true. You have not reached that point yet but you can practice it even now. You can take Jesus by the hand and leave them in His hands, meaning your parents...If not the greatest charity wanting Jesus for someone, God's Will for someone? Follow Jesus and that is the best way to encourage them to follow Him as well.
Start by talking to your parents and to a good Priest and Jesus will do the rest through the intervention of His Mother. Jesus could of said the same thing as you after all, He is the Single Child who did not want to leave His Mother alone, and make sure His foster-father Jospeh was alright. God took care of the Holy Family too. Traditionally, Joseph passed away, and Jesus left His Mother right when She needed Him the most according to human standards, but not according to the Father's Will. We have so much to learn about real, true, divine love. God Father knows what's best for your Father, the Mother of God knows what's best for your Mother and you want to obey Jesus and the Father's Will, through Mother Church.
My Father died when I was seven years old and I felt obliged to stay with my Mom as long as possible(God rest her soul, she passed away a few months ago with Dementia and yes, was cared for by strangers, but good ones, God arranged everything) I told her of my fears, guilt, anger and desire and she was able to tell me to go to the Seminary and not stay on her account. She could not bear to live knowing she was holding me back from becoming a Priest, though to see me leave would kill her...She was able to give me for the sake of a greater love. I gave once to God before you saw the light of day she said, since the Doctor wanted to abort me, I give you now to God once again. You are not mine, you are His. Thank God for a true Mother's heart. I pray you will go through something similar in your own way but I know you are coming from a convert's perspective in maybe a family that does not accept Religious Life in the first place. God knows that too. Only love matters. They love you, your parents, they would not want to see you miserable on their account. I will pray for you and I'm sure you'll know what to do. No worries.