What to do?

So lets say there is someone very special in your life and you've already made many plans for the future..you love them and want all good things for them. Then they give you this news that is life changing for them and makes their dream come true. They are very excited about it and you are to..then you realize after thinking about it, this changes all the plans for the future. Those plans can no longer happen. This also takes this person away from you. You want to be happy for them but you can't stop crying because its bad news for you. You also tell this person everything that comes into your head but this time is different. You feel like you can't tell this person how you feel because it will ruin the joy of the news for them and make them sad. But you have to tell this person how you feel because its killlllling you and you can't sleep, can't stop worrying, are anxious and cry. Is this selfish to not want your future plans to change? Should you even tell them that you hate this "good" news? or keep everything to yourself?