Anything is possible. You're the one doing the forgiving regardless of what the other partner wants to do. The would have to deal with their decision on their own. It's b/w your partner's faith and what he does in his life that he would have to answer for.
Christianity encourages forgiveness. It is not always about us but the other person. When you're ready to accept the situation and realize that your spouse is also struggling on their own path in life, you may have compassion that comes from that understanding.
You can not change anyone nor save anyone. That individual must arrive at the decision to change and work towards it alone. It your partner does not want marital counseling and is seeking separation, that is the fact you're struggling with.
Unpacking Forgiveness: Biblical Answers for Complex Questions and Deep Wounds by Chris Brauns (Paperback )