Can a human ever be happy in the afterlife while not loving God? I'm talking about the afterlife cause you can easily experience moments of happiness during your life on earth, with any kind of attitude towards God.

But after we die, do we have any other choice than to love God? In other words, is happiness possible without loving God? Heaven is only for those who embrace God, right?

I know many people hate God, because they don't like the world that God created. Otherwise they are no bigger sinners than God-loving Christians. I don't know what would have to happen for them to stop hating God. Probably God would have to explain himself to them, which I find hard to imagine... What will happen to them after death?
First of all, Christianity doesn't teach that some get to go to heaven because they are "smaller sinners" than everybody else. Likewise those in hell are not their because they are "bigger sinners" than anyone else. In fact as humans we all stand on pretty equal ground as far as that is concerned, and if that were the standard we'd all be in hell.

Secondly, men do not hate God because of the world he created, they hate God because he is good and they are not. However consciously or unconsciously, they hate him because he is altogether different than they are. They do not understand him and if they did they would not like what they found. The Bible teaches that men love darkness because their deeds are dark and that God is light. (John 3:16-20) Men will naturally hate that which brings judgement to their deeds. God created the earth "and it was good", there would have been no reason to hate it except for the evil deeds of men that have polluted it. A truly Christian perspective on "hating the world" would involve hating the pollution of sin in it and therefore hating ourselves which would turn us to loving God.

Another point that comes up in your question is the issue of choice. The Bible does not speak of any opportunity for new choices to be made after death. Even though everything will be laid bare and there will be no longer room for doubt or question, the choices are made here. They will be sealed there. Those who (through God's intervention in giving them a new heart) chose to hate sin and love God on earth will continue to do so and be truly happy in his presence. Those who have not been redeemed and hate him here will continue to do so for all eternity even though his nature is revealed.

And to bring all this round to the core of your question: No, it will not be possible for a human to be happy in the afterlife while not loving God. There is nothing else in heaven to be happy about. The lamb on the throne is the central theme in the description we are given of heaven. Heaven is heaven because God is there. It will be unquestionably happy for those there because they will love him (even more than they do now). For those apart from him there will be no happiness at all, only weeping and gnashing of teeth.