There are a number of different ways to write internal dialog. I mostly use italics to represent internal dialog and then eschew using italics for anything else in that piece, if I can. For example:
"Hey Donna, how's it going?"
Please say fine and keep walking.
"Oh, Jill, just terrible. It's Mike again, I think he's cheating on me."
That's rough but you see what I mean. I think I put in a lot of internal dialog intending to show my character's thoughts, as a way of getting inside their head. That's fine for first drafts and second drafts but as I polish a story, I'm mostly cutting the internal dialog out. You don't always need it. I think a better way to show a character's true feelings is through their actions. Let me try an example again:
"Hey Donna, how's it going?" Jill said. She kept her feet moving, stared straight ahead and froze a very non-committal smile on her face.
"Oh Jill, just terrible. It's Mike again, I think he's cheating on me."
Donna's smile mostly hid her wince.
Again, rough, but that's a different way of showing internal emotion. Now I actually would want to go back and add back in one piece of the internal dialog I cut. The 'damn it'.
Internal dialog works best when it is in contrast to what is being said. That sets up a nice, ironic tone that works well, I think. You shouldn't have your internal dialog just echo what the character's spoken dialog just said.
Use internal dialog to show inner conflict that the main character must keep hidden. Use it to expand on what the character just said. Use it to contrast what is being spoken and what is being unspoken.
Now I may change my mind about internal dialog, some stories it may work better in than others and I may go back and re-read some of my favorite authors and find that they're using just fine. But for now, I'm going to say you should only use it a last resort.
Try avoid multi-paragraph soliloquies. See if you can show the same emotion in action. But if you can't avoid it, feel free to use it. It's just another tool in the toolbox.