You can either text your girlfriend and tell her that you're gonna break up with her or tell her in person.
Telling your girlfriend in person is much better than texting her that you're breaking up with her.
When telling her about the breakup just cut to the chase and say something such as (I really loved being with you but I don't think our relationship is gonna work out and I now have a new girlfriend that I find more compatible.
I'm sorry that our relationship has to end this way but It's for the best.)
She may be hurt for awhile but eventually she'll get over it and find a new more compatible boyfriend.
Believe me I've been hurt by breakups but eventually got over the breakups and found a new boyfriend who's now my husband.