What makes it "thick" is the presence of sperm. You're probably producing low. You can not cum for a month and see if that helps. Do more strenuous manly things, like lifting weights or chopping wood in that same period to stimulate testosterone and thus sperm production. If that doesn't help after 4 weeks there's not a lot you can do about it.
If your girlfriend ever wants kids you should probably get your fertility checked btw. No use wasting her time if you're shooting blanks.