Tenses in reported speech

At school we were taught that the sentence She said "I love you" , being converted from direct speech to reported speech form, sounds like She said she loveD me .

The verb in the second sentence stands in Past Simple instead of Present Simple: loveD . The teachers emphasised the relevance of this rule because it contradicts the logic of our native language, Russian.

Although this rule of changing tenses in reported speech seems fine to me (she said she loveD me then, but it doesn't mean that she is still loving me, I still feel like using the 'incorrect' tense in some cases.

It is when the thing that was spoken out was definitely CONSTANT or obviously HASN'T CHANGED by now. For example,

At the first lecture the professor told us that Mathematics isn't easy. instead of wasn't easy [because it is as difficult now as it was then and as will always be!]
I told her that I am a vampire only yesterday. instead of I was [because I couldn't have changed within one day!]

Is it correct?

I also feel many times that reported speech rules should simply not be there, for the same reason as mentioned above, namely, that if something is constant in nature, I feel it's awkward to refer to it in the past.

The professor told us: "Mathematics isn't easy".
The professor told us that mathematics isn't easy.

If I said "wasn't" there, it would sound to me like mathematics wasn't easy when the professor told us so, but it IS easy now.
Instead, if I use "isn't" the ambiguity is gone, and it sounds to me like mathematics wasn't easy when the professor told us so, and is still not easy.

Of course, I know all about reported speech tense change rules... But it's perhaps the only part of language grammar (it's not just an english thing, it's the same for many other languages as well) that I really don't like!

Dinosm, do you mean that reported speech rule doesn't sound good to you in this example, either, but it must still be applied? Did I get you right?

Yes, "the professor said that mathematics wasn't easy" is the grammatically correct form and it is what should be said if you want to speak and write correctly.

What I meant above is that to me personally this structure doesn't sound logical, and it is the only part of grammar that I would (and most of the time do) misuse on purpose!

Yeah!
Beatles!
She said she loves you, and you know that can't be bad! She loves you, and you know you should be glad. She loves you! Yeah Yeah Yeah!
Great, maj! If someone accuses me of ignorance for using Present Simple in this case, I will refer to The Beatles