Dude I am super close to being in the exact same position as you. I'm staying with my friend, started learning German here, and have barely spoken a word despite living with him for over a month now. But I think one of the huge things to learning German is speaking it, even if you only know a little bit.
If I've made any mistakes (which I definitely have) please correct me. Ich kenne nur ein bisschen Deutsch ;O Without further ado, here's some tips to help speak in German when you're surrounded by English (that are helping me)
Do you know how to say the thing you're about to say in German? Say it in German. Friend asks what you're doing? Ich koche Frühstück [I'm cooking breakfast]. Are you cooking for both of you? Casually say "Frühstück ist fertig" [Breakfast is ready] when it's finished. Small things count. I'm sure your girlfriend will be impressed by your initiative :)
If your girlfriend asks what a certain word means in English, ask what that word is in German. Download an offline dictionary to your phone so when you are out you can look up words on signs or words that you here. Oftentimes when my friend is speaking with his friends in German, I'll pick out words that stand out to me to look up.
If they say something in German but you can't understand what they said because it was too fast, ask them to say it slower. For example, my friend and I were speaking in English and says there's a saying in German, "Stille Wasser sind tief". He said it very quickly so all I got was something to the effect of shtilahwassernddif, but when he said it slower I got all the words. Asking people to say things more slowly will be your go to thing for a bit. It'll be annoying but it helps a lot.
Do you feel like speaking in German? Ask "Können wir in Deutsch sprechen bitte?" [lit. can we in German speaking please] If they start off speaking really quickly (my friend automatically starts at native level speed haha) "Kannst du ein bisschen langsamer sprechen, bitte" [lit. can you a little bit slower speak please]. Don't be afraid to ask them to speak slowly!!! Seriously!
And yeah, that's all I've got for now. If you're looking for some place to practice, Hello Talk is a great app for writing and check out your local university to see if they have any tandem English German language partners. Viele Glück!