Matty.....First of all, it's TOO much......not to much.
Better you think long and hard, take your time and weigh options, than not give something enough time and brain power. You claim you do this with "simple" things, but who is judging whether they are simple or not? Are they simple to you or are you being directed to think they're simple issues yet you find them serious or major issues that require much thought?
True Matt, most situations we deal with in life on a daily basis, ultimately end in a positive light. However, we don't know this for certain unless and until it does happen. If you can admit that you are "overthinking," you can also do something to curtail this obsessiveness. Let your girlfriend help you by opening up a discussion about anything that is totally unrelated to the topic that is invading your thoughts.
It also helps to get involved in activities that requires use of your mind more than physical. Word games, crossword puzzles, logic puzzles.....whatever you might like. A game of scrabble is always a good way to keep your mind focused on fun rather than problems.
If you practice at this repeatedly, it becomes easier and more automatic. I believe eventually you'll get a good handle on this Matt. Get more exercise and fresh air also....and eat healthy. Keeping your body healthy always help our mind. Good Luck.