The 144,000 are all Jewish men, virgins, who come from the original twelve tribes and are sealed during the Great Tribulation (Rev 7 & 14). These are 144,000 Jews who during the Great Tribulation (the time of Antichrist) will accept Jesus as their Messiah. The rest of the Jews will actually accept the coming Antichrist as their "messiah".
The "little flock" Jesus speaks of is the Jews in Jesus' time that accepted Him. The "other sheep" are the Gentiles of then and now who accept Jesus as their Savior.
Everlasting life is granted to all who accept and believe in Jesus Christ (John 3:16). Nowhere in the Bible does it say that only 144,000 recieve immortality or that only 144,00 will go to heaven. Heaven is open to all who accept Jesus as their Messiah (John 14:2-3, Heb 11:16, 2 Cor 5:1).
Revelation 7:9 tells us that there is "a great multitude, which no man could number" in heaven. How could it be numbered (144,000) and then not numbered (great multitude)?