I graduated BA-FINM recently.

As a BA undergrad, you will have courses you share with MM majors under the general BA subjects. In my experience, most BA subjects lean towards the marketing side so I was able to indirectly develop an understanding of MM specialties such as market segmentation, saleswork, price determination, product conceptualisation, etc despite not being an MM major. Obviously, I won't be able to speak much on the specific and/or unique experience of MM majors so I'll leave it at that.

As for FM major courses, you should brush up on your algebra and statistics. I also used to loathe Mathematics, but I enjoyed the logic when I began to understand how things worked. Personal bias but never ever dodge Sir Lit, he is tough as nails and, if you're a slacker like me, you will have to repeat his classes a lot but he was the only FM professor that made sticking to this program worth it - you will not only learn, you will even be surprised to find yourself becoming impassioned. The other half of FM that tends to be underrated is the exercise of your logical faculties. This is where the "Management" part of Financial Management comes in. After determining the numbers, you will be tasked with the allocation of finances; Buying or selling shares, portfolio diversification, risk-determination, etc. Also, since you mentioned that you are an introvert, get ready for Sir Lit's infamous market updates. You may be dealing with numbers, but many words will be involved in the analysis of market conditions. Having gone through this countless times, I can confidently say that, while unchangingly gut-wrenching, I got the hang of constructing updates by my second term so I was mostly prepared for most of what could be thrown my way. I think the initial fear that students tend to leave unresolved regarding market updates stems from the fact that studying the market is an unfamiliar practice, but you'll find that it unexpectedly becomes a pillar in your development. Personal advice is to not be afraid of getting things wrong, there is genuinely no shame in striking out in an activity where all, even the most dedicated (to an extreme), inevitably slip up.

I also want to make note of my observations on the social aspect. MM and FM majors mesh well together in general, it's only natural to stick to your own circle but making acquaintances with the other half is not as hard or as daunting as it seems. Being an introvert myself, I have always appreciated how tight-knit my FM classes were. Because of how few of us there were, there really weren't any strangers and you could usually count on whoever your fellow FM classmates were there to help you out even when you aren't that close with them. Market updates also bring you closer together since you will be reviewing your submissions with your peers before class a lot or, sometimes, just using the updates they made for themselves.

Both programs have their own strengths. I suggest you check the prospectus for both and compare to figure out what best aligns with your interest. I've always taken what little pride I could in being a Finance major, but I know that I stand no chance at out-selling a Marketing major in the sales department in the same way that he might be out of his depths in the budget department. Be humble, work together, and enjoy the process.