If you loan your significant other money, the best mentality is probably to just assume you won't get it back. That way, if you get it back it's a nice surprise and if you don't, you're not going to be disappointed or resentful. Unless, of course, it's a massive amount of money and then maybe you should get a lawyer involved. Treating people you love to dinners, vacations or other gifts is a nice gesture and keeping tabs on every penny isn't always healthy for every relationship.
That being said, if you do make an agreement with your partner to lend money and get it back, is it ever a good idea to forgive that loan as a birthday gift? This is the exact problem that a woman on Reddit's (Am I the As*hole) found herself in. Probably should've sprung for the Airpods...