This is the sentence that you want to use:
I'm so happy to be here.
This sentence should normally be avoided:
I'm so happy for being here.
I can think of no real syntactical reason why for being is wrong, but it's not idiomatic and would not normally be used. (I've never encountered it before, but that doesn't mean it hasn't been used by somebody, somewhere.)
Unfortunately, I can't give a better answer than that.
I'm afraid it gets even more confusing.
Some things simply don't pair well with other things. However, it appears to mostly come down to common usage and what is used and sounds natural rather than there actually being specific rules in every case.
Following are all examples of things that do and don't sound right. If you look only at the syntax, there appears to be no objective pattern.
✗ I am happy for being . . .
✓ I am happy to be . . .
✓ I am sorry for disturbing . . .
✓ I am sorry to disturb . . .
✗ I had thought for visiting . . .
✓ I had thought to visit . . .
✓ I listen for singing . . .
✗ I listen to sing . . .
✗ I try for singing . . .
✓ I try to sing . . .
✗ I have tried for seeing . . .
✓ I have tried to see . . .
✓ I am crazy for trying . . .
✓ I am crazy to try . . .
✗ I am struggling for thinking
✓ I am struggling to think
I would be happy if somebody else could come up with a more coherent explanation for all of this rather than "it just sounds right or wrong."