I know what you mean. Cherish the “remaining days” of just the two of you. Take lots of videos and photos. Tell your firstborn many many I love you. And even if he/she doesn’t understand yet, bring his/her palm to your belly and say together, you two will love the baby.
You’re a good mom for having that “guilt”. Maybe prep yourself mentally that you will continue to have a good emotional management when the newborn arrives. Many parents lack patience towards the older kid when situations is stressful.