Are ‘very energized’ and ‘as I told myself’ correctly used in the sentence?

Yes.

Aside from the fact that the "very energized" sounds a bit awkward here, it's fine grammatically.

If I rewrote the sentence as follows, would it still be acceptable, teechr?

When I read Grandpa’s letter of advice, I felt very energized as I told myself, “If Grandpa were here by my side, I would be much happier.”

RommelWhen I read Grandpa’s letter of advice, I felt very energized as I told myself, “If Grandpa were here by my side, I would be much happier.”

The sentence itself sounds a bit funny/forced, but the grammar is fine.

XerxesThe sentence itself sounds a bit funny/forced,

How can I make the sentence sound not a bit funny or forced, Xerxes?

RommelHow can I make the sentence sound not a bit funny or forced, Xerxes?

Give it your best shot, and I will tell you if you've succeeded. Your original question had to do with grammar, and in your sentence the grammar was fine.

RommelHow can I make the sentence sound not a bit funny or forced, Xerxes?

This is just an example, Rommel:

Reading Grandpa's letter cheered me up. I told myself, "I would be so much happier if he were here with me".

Now I see. Thank you, Xerxes.

RommelNow I see. Thank you, Xerxes.

You're welcome, Rommel.