I need to break up with my girlfriend.
Why do you want to break up with your girlfriend?
She massively disappoints me every time we're hanging out.
If you feel that you would be happier not being in the relationship, then this is a good reason to breakup. However, trying to talk with your partner could be something that fixes this issue. May be a good idea to run it by your parter and tell them why you are being dissapointed.
It's hard for me to talk about it, but I have a job in another town and I don't have time to visit my girlfriend. I have a new potential girlfriend which I adore and is perfect for me, but I need to break up with my current girlfriend so I can date her.
Well it does sound like you don't want to be in the relationship at all with your current girlfriend so it would be a good idea to leave her. Especially if you are interested in a new girl.