I used to make fun of other people and done some backbitings too. Now after knowing the bad consequence of it, I feel very afraid. It is not possible for me to go to each and every of those people and seek for forgiveness. Now I know that until they forgive me, all my good deeds will be gone. This makes me feel very afraid. :( What should I do now? :(
One great thing about you is that you realize the significance of what most people consider as a "small" sin. You take this "small" sin so seriously up to the point where you are afraid of the consequences, especially in the after life. Thus, you decided to take on the path of repentance seriously. At this point, I believe you are already on the right path.
Now for you question. Here's a spot on answer from islamqa.info (Expiation for Gossip):
What the one who wants to free himself of the sin of gossip must do is strive hard to seek forgiveness from the one whom he gossiped about, and ask him to pardon him, and apologize to him with kind and good words, and he should be as humble as he can in this, even if he has to buy an extremely valuable gift or offer financial help. The scholars have stated that all of this is permissible when it comes to restoring people’s rights.
Because the scholars among the righteous salaf (predecessors) and fuqaha (jurists) thought that seeking people’s forgiveness for gossip might lead – in some cases – to greater evils such as grudges or breaking of ties, and people might feel resentment and grudges to an extent that is known only to Allah, most of the scholars granted concessions allowing one not to seek forgiveness (from the victim), and they hoped that it would be sufficient to pray for forgiveness for the victim of gossip and make du’a (supplication) for him and praise him in his absence.
Other scholars were of the view that nothing could expiate for gossip but the forgiveness of the one who was wronged. But the correct view is that if the one who gossiped repents sincerely, he does not have to tell the one about whom he gossiped about it, especially if he fears that this would cause more trouble, as is usually the case.
Regarding the du'a itself:
Undoubtedly such a du’a requires us to specify the person for whom we are praying, either mentioning him by name or describing him, by saying: O Allah, forgive me and the one whom I have gossiped about and wronged; O Allah, pardon us and him, and whatever else you can say in your du’a.
As for praying in general terms, it does not seem to be sufficient to achieve what you are hoping for from Allah. Just as you gossiped about him by mentioning his name or describing him, and you singled him out for harm, so too you should pray specifically for him and ask for forgiveness for him, so that the bad deeds will be replaced by good.
In short, you need to repent sincerely, never go back to backbiting, strive hard to ask for forgiveness whenever possible, and finally, make sincere du'a for the person you're backbiting. That should be sufficient to remove your anxiety, keep moving forward in the path to repentance, and in the end, receiving forgiveness from Allah.
Allah knows best.