Is it sinful of me to feel jealous when my husband marries again?

No — jealousy is natural and not a sin as long as it does not lead to wrongful actions. The Prophet's ﷺ own wives felt jealousy; Sayyidah Aisyah r.a. once broke a dish of food sent by her co-wife, and the Prophet ﷺ only said gently, 'Your mother is jealous,' and replaced the dish. What must be guarded is your actions: do not backbite, slander, cut ties of kinship or neglect your responsibilities. Acknowledge the feeling, express it to your husband wisely, increase your supplication and patience, and seek counselling if the emotions are too heavy. The husband, for his part, must be gentle and understanding, not belittle his wife's feelings.