@JoeJ Yes, with shaders is the best solution, but I never used them, and it seems a little hard. I tried to understand some open-source applets with SM and tried to apply to my game for some days, but nothing. :(

What part is hard? The math and algorithm? Or the API side of loading / compiling shaders, not knowing what's wrong because nothing happens, etc.?

I'm not 100% sure, but switching to shaders probably forces you to implement the entire lighting yourself in pixel shaders. So that's more work than initially thought. :/
But then you can work on per pixel lighting first, which is easier than shadow maps. After that you may be experienced enough.

If you're finding it difficult to pick up shader-writing from code, then perhaps you might find it easier via tutorials. Something that can take you one step at a time through the matter, rather than a program already full of unfamiliar keywords and concepts.

I don't have a specific tutorial to offer, but I imagine that there are some out there to be found!