My spouse says they love me but are not in love with me anymore. How can I get them to love me again?
You can't. The harder you try to make your spouse love you, the more they will resent you and move away from you.
It is common for an affair spouse to make the "I love you, but I'm not in love with you" declaration during or after an affair. It is one of the most common phrases I hear. I understand your deep desire to be wanted but don't get caught in a cycle of desperate attempts to make them love you again. Their failure to love you has more to do with their deficiencies, not yours.
The best thing you can do is invite your spouse back to your marriage and then leave him/her to their choice while you figure out how to move in healthy directions for you and your family.
Suppose your partner's uncertainty causes them to disengage from you or threaten to return to the affair. In that case, the best thing you can do is honestly and clearly communicate what you want ("I love you, and I want this relationship to work..."), establish your boundaries ("I will not cooperate in a relationship where I am sharing you with someone else..."), and set your spouse free to make his/her choices. You cannot control what they do. The harder you try, the more certain the failure will be.
The only way genuine love will return is in the context of respecting you. The question they struggle with needs to shift from whether or not they want to stay with you (the question they ask when they are in control of your responses) to whether or not they want to lose you (the question they ask when you are in control of your responses).
Many couples get stuck in this place of confusion and indecision. If they don't find their way out of this state, they will likely do more damage to their marriage or relationship. Your partner may need to find help in moving through their next choices, but you cannot force them to get help or make their choices for them.
Note: One entire section of the Affair Healing for Betrayed Partners Course (Part 4: “The Uncertain or Uncooperative Spouse”) is dedicated to providing specific guidelines for responding to a partner that is uncertain about their choices.