I can't understand why he ignores her and what he means when he speaks about only helping God a lost sheep and taking food from children to give to dogs?
Matthew 15:24 But he answered and said, I am not sent but unto the lost sheep of the house of Israel.
Jesus came in fulfillment of prophecy. Jeremiah foretold of a new covenant that would be made with the household of Judah.
Jeremiah 31:31 Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:
Israel was given every opportunity to receive this new covenant. Jesus didn't wink and tell his disciples, "I know you guys are going to blow it, so here is the real plan".
The exclusion of gentiles at this point was not a slam against gentiles, but a focus all on Israel. Israel would not be able to say that when the Messiah came he was distracted by ministering to gentiles so it wasn't really their fault that they didn't receive the Kingdom when it was offered.
We can see this same theme played out even towards the end of Jesus earthly ministry.
John 12:20-24 And there were certain Greeks among them that came up to worship at the feast: The same came therefore to Philip, which was of Bethsaida of Galilee, and desired him, saying, Sir, we would see Jesus. Philip cometh and telleth Andrew: and again Andrew and Philip tell Jesus. And Jesus answered them, saying, The hour is come, that the Son of man should be glorified. Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit.
Jesus came for Israel first. He would have been ruling on earth for the last 2,000 years if Israel had been faithful. We get a little picture of this as Jesus gives instructions to his disciples for their role in ruling the twelve tribes and the expectation that they would go to the whole world.
Mark 16:15-18 And he said unto them, Go ye into all the world, and preach the gospel to every creature. He that believeth and is baptized shall be saved; but he that believeth not shall be damned. And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.
At the first covenant, the entire nation of Israel agreed to the covenant with God.
Exodus 24:3 And Moses came and told the people all the words of the LORD, and all the judgments: and all the people answered with one voice, and said, All the words which the LORD hath said will we do.
At the offering of the second covenant, not many were interested. Even when the miraculous signs and wonders were performed, Israel was still not interested. Even when gentiles were being given the powerful gifts of the Spirit as proof of their salvation, Israel still was not interested.
There is coming a time when a remnant of Israel will be faithful and will receive her King and all of the prophecies of Israel will still be fulfilled.
I think the fact that Jesus gave the gentile woman her request is a result of her persistent faith similar to the parable of the woman and the unjust judge (Luke 18)