Cancel on the future gift part. Your verse number in the Douay Rheims is different from other Bibles.
It doesn't negate the point though. Jesus said "THIS IS" He spoke in the present, AND he spoke in the future. When he spoke in the present He was speaking symbolically for it wasn't His physical body. It represented what WOULD HAPPEN.
Now, I await your answer to my question about what would happen if I didn't like the thought and refused to partake of Mass.
They weren't physically eating Christ's flesh and drinking his blood physically when Jesus said "This is..." It was symbolic then
In John 6 Christ indeed describes a future gift of His body, without giving them anything to eat at that time. But that does not make it symbolic; His crucifixion also happened in the future relative tot he discourse and that was not symbolic. They take it correctly as a prediction of eating His body in the physical sense, get appalled and leave. He does not correct their impression, but rather reinforces it with several amens, the "food indeed" and the choice of the verb as "trogo", literally "gnaw", at one point.
Christ offered one sacrifice for sin and there remains no more
That is correct (check Hebrews). This one and only sacrifice occurs every mass though.
works do not save you
This is extrascriptural spin. Read James 2.
If I were a Catholic but did not participate in mass, ever. I didn't like the thought of eating Christ's body and drinking his blood. What would become of my soul when I died?
Your soul would then be in great peril because you would have wilfully rejected Christ:
57 He that eateth my flesh, and drinketh my blood, abideth in me, and I in him. 58 As the living Father hath sent me, and I live by the Father; so he that eateth me, the same also shall live by me.
, and violated His commandment:
Except you eat the flesh of the Son of man, and drink his blood, you shall not have life in you.
You are in the same peril, incidentally, whether you consider yourself Catholic or not, because the commendment comes from Christ Himself and not from the Catholic Church.