Now that my friend has a boyfriend she never wants to hang out.
Is asking her to hang out something you have recently done?
Yes, and she always says no because she's with her boyfriend.
It's rude of your friend to not want to hang out now that she has a boyfriend. Ignoring your friends just because you're dating someone is really inconsiderate, because your friends will be there for you no matter what happens.
This situation makes me upset and gives me a desire to treat her like shit.
It's upsetting and rude that your friend is ignoring you now that she's in a relationship, however, that doesn't justify you treating her poorly. Talk to your friend about how she is being distant from you and explain that you are feeling neglected as a friend. She might not even realize that it is something that she is doing.