is the following sentence grammatically correct?

There's no such thing as a guidance, because guidance is a verb, not a noun.
You could say "You have always given me guidance."
You could say "You have always been my guide."

But you can't say, You are like a guidance.

Just a minor point - advice doesn't get an 's'. The word advice works for singular and plural.

I think it's just "advice" and not "adviceS"

No, "advices" should not be plural.

While the word 'guidance' can be used as a noun, it is not usually used in such a context. A better word would be guide.

The word advice is not used in plural form. It should just be '...give me advice when...'

ummm no. but try this;
"You guide me & are always there to give me support & advice when i need help."

No it isn't.

You are like a mentor to me; you are always there providing support and giving advice when I need help.

I would edit it myself to "You are my guiding light. You are always there to support me and counsel me when I need it most."