God is not trying to punish you or make things more difficult for you by limiting sex to marriage. He is protecting you physically, emotionally, and spiritually.
You are already vulnerable emotionally because of the separation or divorce. A sexual relationship can cloud your judgment, setting you up to be hurt. Also, deliberately disobeying God will block the joy, peace, and assurance that come from Jesus.
Physically, there are many consequences to having sex outside of marriage: sexually transmitted diseases (some of which can be fatal), pregnancy, and being forced to do something you do not want to do.
Rose Sweet explains the spiritual importance of sex: "Some might think God is being unreasonable to limit sex to marriage, but that's because we're ignorant. Most of us haven't really tried to understand the fullness of what sex is. Sex is a renewal of the marriage bond. It's a renewal of a covenant—just as God was with His people in the Old Testament and as Christ is now with the church. In other words it's saying, 'I take you forever, completely, and in every way, and I will never bring any harm to you.' You can never have that in a casual or in an emotionally committed but non-married sexual relationship."
Choose to stay within God's limits. If you are not sure what God's limits are about specific acts surrounding the sexual experience, search God's Word for principles that apply to those situations. Read Ephesians 4-6 together.
"This is love for God: to obey his commands. And his commands are not burdensome, for everyone born of God overcomes the world. . . . Who is it that overcomes the world? Only he who believes that Jesus is the Son of God" (1 John 5:3-5).