(New to DND/Roleplaying) How to better RP A character struggling for power from an evil entity?

As stated I am fairly new to the game and have roughly six sessions under my belt in our campaign and two one shots and a guest appearance on another campaign. Mechanically I understand the game but I am fairly new to roleplaying….and clearly didn’t consider that before designing my first character. I won’t lie and say I made the character after understanding how the game worked…however our DM was very open to homebrewing something fun for me.

My character is a female half elf Hexblade Warlock. Brief backstory summary is her family and lineage is cursed for using magic in search of power. Saya (my character) was forbidden to use magic from birth in fears of the curse, she naturally uses magic and becomes interested in it. Her parents are killed by the fiend that cursed them for trying to stop her from using the magic that makes it more powerful blah blah blah flavortown. Her sword at some point in her search for this fiend become imbued with his power unbenounced to her. During a hard fight as she went down a voice appeared in her head as she blacked out asking if she wanted to live and have the power to do so. subconsciously she agreed (making her pact with the fiend and becoming a hexblade) she awoke with small horns (homebrew for later) frightened by then she hides them with a witches hat she finds in town before meeting with the rest of the party, keeping them hidden from the party.

Saya is NOT an evil character. The FIEND however IS. Her homebrew properties are her horns, they are designed around a custom wild magic table our DM made for her. There are 10 levels to the table increasing every time she rolls the level number or lower on cantrips or spells. Anytime this happens a random wild magic event tailored around her happens….as well as her horns growing an additional inch. Each time the fiend slowly gains power as Saya has to fight it off, eventually losing control temporarily or possibly permanently becoming feral and attacking anything in sight (including the party)

Now my question is….being new to roleplaying, how should I go about this journey? It is a struggle for power between her and fiend, the closer she gets to 10 the more….evil? I feel she will become before snapping. It is already a challenge for me to have picked a female character ( I am male) but to have also made a character with seemingly split personality issues as my first character.

note the rest of the group is VERY talented at roleplaying so I am trying extremely hard to do my best at this and want to be as interesting as possible with how amazing all of the other players are.

Thanks in advanced!