Sources close to Melanie tell TMZ ... she is still very much mourning the death of her fiancé, and that this first Christmas without him is, no doubt, going to be incredibly difficult -- especially since last year's was so full of joy as they shared time with their son.

With that said, we're told Melanie is going to pay tribute to her son's father in a variety of different ways during the holiday -- via gifts, decorations and even a home-cooked meal.

For starters, we're told Melanie has put together a scrapbook full of pictures of Aaron, herself and their kid ... which, while not totally complete, will be placed under her tree for him. Speaking of presents, she also got something for young Prince -- with AC in mind.

It's a child-sized dirt bike, which Melanie gave to Prince early this year ... and which has a direct tie to Aaron -- who loved to ride himself. Our sources say Melanie believes Aaron would've given Prince something like this at some point ... so no better time than now.

On top of that, we're told Melanie was gifted a custom ornament in honor of Aaron that she's putting on her tree as well -- a wing-shaped trinket with Aaron's name engraved on it.

There's also the matter of food, which we're told was big for Aaron around this time of the year. In 2021, our sources say he cooked up a feast for Melanie and her family -- which included Kobe beef ... so this year, we're told, she's preparing a similar meal as a tribute.

As we reported ... Melanie has been able to regain custody of Prince in the wake of Aaron's sudden passing -- and it sounds like she's doing everything she can to cope with his loss.

He was 34.