Why did God choose Israel, and not another nation? A simple answer is, because God had to start somewhere. God knew that after Adam and Eve sinned, their descendants would plunge into ignorance and darkness about the truth, about who He is, about every essential spiritual detail. Satan had blinded and hardened people's hearts and minds about God even by the time of Noah.
However, both before and after the flood, there were a select few people who still believed the truth about God and the promised Messiah, which was passed down to them from Adam and Eve. These included Abel, Seth, Enoch, Methuselah and Noah and his family. After the flood, Noah still taught the truth about the true God, and the promised Savior, but down through the generations, the truth was forsaken once again. Still, however, a select few retained the truth, and Abraham was one of them. It would be through the line of these who had retained the truth, that the Messiah would be born.
The nations of the world had resided in Asia, Europe and Africa, and the Middle East was the most strategic geographic place for God to begin proclaiming the truth about Himself and His Anointed One who would save humanity. Palestine was a highway through which people from all other nations had passed, traveling for business or personal purposes.
And while they travelled through this area, they would hear, by Abraham and his family, the truth, the "gospel", proclaimed.
Just as children easily learn spiritual things through object lessons, so the whole Old Covenant was a physical object lesson of the spiritual reality of the Messiah and our salvation. The animal sacrifices, practiced from the very beginning, the sanctuary and its services, the feasts of Israel, were all symbols of salvation. So just as the physical nation of Israel was God's chosen nation in the Old Covenant, so the spiritual Israel, the church, those who keep God's commandments, out of all nations, are His true people.
If you carefully study Exodus-Deuteronomy, God made provisions for Gentiles as well as Jews in the sanctuary services and in the feasts.
It was the Jews of Jesus' day, and even today, who became selfish, bigoted and ethnocentric, claiming that God only loves them since He chose them, and hates all other nations. Pure garbage! For God so loves the WORLD!!
They hoarded the treasure of salvation for themselves, and didn't want the rest of the world to share in their wealth of knowledge of the true God, and of their salvation. This is where many are deceived about God, thinking that God shows favoritism. He does not.
There are plenty of verses in the OT where God shows acceptance to Gentiles. Rahab and Ruth were ancestors of Jesus, and they were Gentiles. God promised blessings to the "eunuchs" and the "sons of the strangers" in Isaiah 56 who would keep His statutes and His covenant.
It is very politically incorrect today to say that Israel is not God's chosen people anymore, because people will then jump the gun and say that you're anti-semitic. Actually, those who continue to lead Israelites into believing that their ancestry secures their salvation, being God's chosen nation, are the ones who are anti-semitic, because this belief will lead them into hell. It is not your physical ancestry, neither your spiritual heritage, such as being born into a Christian family, that saves you. It is Christ alone who saves.
I'm sorry that you're getting doubts about this. I hope I've cleared things up a little bit for you. PLEASE don't give up! Pray and ask God for enlightenment. trust that He loves the world, no matter how it appears that He doesn't. He will, in His own time, show you the truth, and the truth shall set you free.