I don't think that Clarissa flees with Lovelace because she loves him. There is little in the correspondence to indicate that she has an emotional connection with Lovelace. She does not confess a deep love or bond to him. Her primary communication with Lovelace starts because she is repulsed by the notion of marrying Solmes. Clarissa is shown as one who recognizes with a stunning lucidity that her options as a woman in the time period are limited. She runs away with Lovelace because these options are few.
Another element reflective of Clarissa's lack of love for Lovelace is that she suspects him. This would indicate that there is not much in way of love. Yet, she takes a chance because she realizes that marriage to Solmes is a guaranteed path of futility and pain. If Clarissa is in love with anything, it would be the hope of a better life. For better or worse, she thinks, at the time, that this rests with Lovelace. When she inscribes the date of her death on her coffin as the day in which she ran away with Lovelace, it becomes a clear reminder that she never did Lovelace. Rather, if she was in love with anything, it was the hope of a better life or one that was not so agonizing for a woman in Clarissa's predicament.