If you're really concerned with it looking like a triangle, you can request that they cut it in a way that makes it point more towards your face. There's a word for this, but I can't remember it because technically I'm supposed to be doing my job and not just watching people get haircuts. Also, you should talk to them about thinning it out. You don't necessarily have to go for a whole lot of thinning, but just taking chunks and hitting them with the thinning sheers can help. Currently, my hair dresser takes sections of my hair, twists them, and hits it in random places with the sheers just to take a tiny bit of bulk out of every section. I'm sure if you go to a good salon they'll be able to tell you what they like best.

Also, you are making me want to cut my hair off again because I'm super sick of it but my hair basically never looks good, so it is a lost cause.