Why do you think Paul said these words?
I comprehend that Paul is passionately expressing his willingness to give anything to see his fellow Jews believe the Gospel and be saved. Many traditionalists view the phrase 'separated from Christ' as being equal in meaning to 'going to hell'. (See also 1 Cor. 16:22, Gal. 1:8-9)
"Many have thought that his meaning is, that he had so great a value for them, that he could even wish himself, and be content to be eternally separated from Christ, everlastingly banished from his presence, never to enjoy communion more with him, or in other words, to be eternally damned, that they might be saved." - John Gill (biblestudytools.com)
Could Paul have really meant that he would choose to 'suffer in hell' in the place of his fellow Israelites? We know Paul wrote inspired words by the Holy Spirit. I believe there is an answer to this mystery.
"Very truly I tell you, whoever hears My word and believes Him who sent me has eternal life and will not be judged but has crossed over from death to life." - John 5:24