Advice needed on keeping X5 or selling

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One thing about cars is they always depreciate and when financing the only way to avoid having a negative equity situation when it's time to trade it in is placing a large down payment or keeping the car for several years. Having shorter (60 mo or less) terms helps.
The good thing is you bought GAP so at least you're not on the hook if something were to happen to total your ride.

I'm with the others that you should look to refi this loan, rates just cratered yesterday with the 10-yr bond slipping below 2% (the cost of funds for most banks). I myself am looking to refi the car I just bought last month because I can get a rate in the low 3% vs the low 4% I have now.

Just be careful that your GAP policy only covers you for this loan so you'll need a lender that offers gap on the new loan in order to maintain that coverage or check if your auto insurance carrier has a "loan/lease" coverage option that is often way cheaper than buying GAP.
Also note that your current GAP policy is rebateable. You have to go to your dealer after refinancing and request they cancel the GAP policy (usually within 30 days after loan payoff) and you'll get a pro rated amount of those funds back.

Finally, if you're having trouble paying for the car please don't spend money modding it. That's Adding more debt to the equation without adding any value to the car. (Yea you'd value it but if the car was stolen or totaled you won't see any of that money). This may not be the F85 you want but enjoy this ride for a few more years then get the next X5M down the road when you can better afford it.