Mental calculation is not 'mathematics'. That is a wrong popular belief. Arithmetic operations (like addition, multiplication) are only a very small part of what constitutes modern 'mathematics'. So, don't say you 'suck' at 'maths', because you likely don't know what you are talking about.
And it's wrong, too, to think you can't do mental calculation. The ability to compute sequences of arithmetic operations with large-ish numbers fast mentally can be learned. It is all a matter of exercise and exposure.
In general, the exposure and the exercise allow you - with time passing - to memorize lots of triples of numbers and the operation that you associate them with. (Like (1, 4, 5) and (+).) Then there are simple algebraic tricks that will allow you to make that very sparse list of numbers denser. You basically get new triples for free by applying those tricks. Say, the square of 13. You don't know it, but you know the square of 12. 13^2 = (12+1)^2 = 144 + 24 + 1 = 169. That was faster than calculating 13^2 "manually", because I basically looked up the components of 13^2, that is 12^2, 2*12, 1^2, mentally which happened almost instantaneously. There are more of those so-called 'identities' - or, for our purposes 'tricks' - to help you.
It really depends on how much time you are willing to devote to practicing mental calculation.
I'm kinda proud of the fact that I'm not good at maths My Dad always told me I would need it in life but aside from coming unstuck in the occasional pub quiz when a maths question comes up, it doesn't bother me. **** mathematics!
How would you feel if someone made openly fun of what you love and exclaimed that it was to be ****ed?