Grief is not linear, you will continue to travel its squiggly lines for a long time.

No one here is doubting your love for her, and Black Cat is never going to replace Melbcat, because she is irreplaceable. But it is okay to share what is left of your heart while it heals with another creature who sounds like he does need some help. It took me a week to decide to bring another cat into our home after I lost my old boy Scooter. 10 days after I lost him we brought home a pair of kittens from a rescue, and that 3 day delay was only because we had covid at the time and had to wait for clearance. The kittens were very young and unwell with cat flu. They required a lot of care, which helped heal my heart and kept me busy. I still miss my giant black boy, but after nearly 4 years I can remember him softly and I hope he forgives me for leaving the decision a day late, and for bringing now 3 other cats into his home. Love is not pie and there is plenty to go around as your heart grows with each addition, not separated by pieces. There is no wrong decision here, just follow your kind and giving heart. If you don’t have the capacity, you will help him as much as you can, and thats enough.