The Nymph's reply is rings of sarcasm and possibly a tinge of regret.
The Shepherd has offered her many things to "come live with me and be my love." However, the Nymph's response includes her pointing out that all of the things the Shepherd has offered her are temporary. All die, dry up, fall apart...nothing he has offered has any staying power. That includes his proclamation of "love". He never mentions marriage. It's more of a "lust" call than a love proclamation.
She says that IF she could be sure that time would stand still and all these flowers and homemade items could last forever with the Spring/Summer months, then she might consider the temptation. However, since she knows it is not true, and he has not offered a commitment past three-four months, she's not biting.