This need not be an argument at all: it's a flaw
Here are some tools for your discussion with the DM
Flaws have a place in D&D 5e that is soft around the edges; there should be room to work with the DM on how this fits into role play. You two need to have an OOC conversation about this.
Your line to take: this new flaw should not be treated as a curse nor a compulsion (like a magical effect) based on what flaws are in D&D 5e.
What are flaws in D&D 5e?
Flaws are by intention a point of departure for role playing that are created collaboratively between the player and DM at character creation.
Basic Rules, p. 8 (same as PHB)
Choose your character’s alignment (the moral compass that guides his
or her decisions) and ideals. Chapter 4 also helps you identify the
things your character holds most dear, called bonds, and the flaws
that could one day undermine him or her.
Flaws (Basic Rules p. 35)
Finally, choose a flaw for your character. Your character’s flaw
represents some vice, compulsion, fear, or weakness—in particular,
anything that someone else could exploit to bring you to ruin or cause
you to act against your best interests. More significant than negative
personality traits, a flaw might answer any of these questions: What
enrages you? What’s the one person, concept, or event that you are
terrified of? What are your vices?
What's different in this case is that you already have a flaw, presumably, chosen at character creation. You have subsequently interacted with, or have been exposed to, some form of eldritch magic that has either added to your original flaw or overwritten it with a new one. (Ravenloft is a dark and dangerous place to play).
But what does a flaw do, in this game? Typically, it provides a way to reward a player for good RP with an inspiration point. In an ideal world, or at an ideal table, role playing this flaw particularly well would earn you an inspiration point, which is something that can allow you to get advantage on a die roll at some point (mechanically) when awarded by the DM based on the DMG's guidance.
Inspiration (Basic Rules p. 35)
Inspiration is a rule the Dungeon Master can use to reward you for
playing your character in a way that’s true to his or her personality
traits, ideal, bond, and flaw.
You can probably negotiate with the DM for how inspiration is awarded. Basic guidelines in the DMG (pg 240-241) says this:
Awarding Inspiration is an effective way to encourage roleplaying and risk taking ... the character can have no more than one Inspiration at a time.
Have a discussion with your DM (before the next session) about how important flaws are at your table if you all don't already have a consensus about that.
Further that point, I have offered you the points that flaws are a role playing feature, not at the level compulsion like curse, for example a cursed weapon (there is one that induces people to attack anything nearby).
Is the reward for role playing well, at your table, the award of a point of inspiration? If yes, then you can choose how badly you want a point of inspiration, or if your internal struggle with all of your other background features may result in your winning your internal battle versus this flaw. Discuss this with your DM and see if you two can reach a compromise. Your role playing can feature your internal struggle with this new thing, depending on how you see that internal conflict playing out.
If award of an inspiration point is not an option, you can rightly point out that you have no incentive to role play your newest/added flaw.
Caveat: this recommendation takes us a little bit into meta gaming territory, and we each have a different tolerance for metagaming.
Is this the best approach to take? Maybe.
We know nothing about the personal relationships and bonds, In Real Life, between you and the people at your table. If you push back on this at the table, only you can estimate how your fellow players will react. Some may support you, some may not.
How critical is it that you meet with and play with this group? If the DM pushes back during your away-from-the-table discussion, and you don't care for the answer, only you know how important it is to you to keep playing with this group. That, I can't advise you on: I am a poor mind reader.
The bottom line
Flaws in D&D 5e are not a mechanical feature, and not a curse, but are instead role playing tools that -- in an ideal case -- is a collaborative effort between the player and the DM.
So, collaborate. I hope you and the DM can arrive at a happy medium.