What goes on in a person's brain is unknowable and terrifying. You can literally think whatever you like and no one will ever know. Seriously though, it's true. Inside your own head you can be as batshit nutso as you like and the only person who can judge you for it is you, and let's be real, while you can be a harsh critic, you can also be extremely forgiving when you need to be. Which is why sometimes, we allow ourselves to think some dark and weird shit.
The thing about our thoughts is that most of the time they're totally transient, and when we're bored or alone they go to fanciful places that don't really mean anything in the scheme of our actual existence. When you're in a relationship, it IS important to curb your thoughts in certain circumstances. For instance, when you're mad, and you want to scream, you have to change your thought process to NOT scream, because screaming at someone you love is horrible. Likewise, you have to change your thought process when it comes to things like sexual attraction to other people, making plans, and generally having someone else in your life. But then there are natural, harmless thoughts you have that don't affect your relationship at all, and you shouldn't feel guilty about having.
1. "I could kill this person."
I mean, don't actually plan to kill them. But the person you're in a relationship WILL annoy you, and they will annoy you to the point you might momentarily think you could live without them. Obviously, that's not the case, and you adore the pants off them, but you shouldn't feel bad about feelings of frustration or annoyance. After all, you're probably annoying them just as much.
2. "I really want some alone time."
It's okay to crave alone time, especially because sometimes that's really hard to come by. It doesn't mean you love your partner any less, it just means that everyone needs a little time, even if it's just ten minutes of silence, to themselves to stay sane.
3. "Or just one night where I can spread eagle in the bed."
Sleeping in a shared bed every night can be exhausting, especially for light sleepers. For us, a sleepless night for your partner is a sleepless night for you too. Don't feel bad if you find yourself, wide awake in the middle of the night, listening to the person you love snoring like a lawnmower, wishing that you could have the bed all to yourself for a night of peaceful sleep. Sleep is important, and wanting it doesn't mean you love your partner any less.
4. "Am I sure this is The One?"
Everyone has nagging doubts, even if they're not "real" or pressing enough to address or end the relationship over. Enduring doubts should probably be spoken about with your partner, but wondering if this is the right person is like worrying if you're buying the right dress, or if you're in the right job: Mostly, it's a passing thought that, in the end, is required for rational decision making, but that doesn't mean you have to unravel your entire world because of it.
5. "I wonder what my life would be like if I'd have made different choices."
Again, these sorts of thoughts are natural. And you're allowed to wonder about what your life would be like if things went differently. Maybe if you had never moved to that city or taken that job or gone to that party, you'd have never met your SO. Maybe it would be worse! Maybe it would be good, but still different. Meanwhile, there's the distinct possibility that there are multiple universes out there, and everything that could happen is happening somewhere, to another you, so wondering what that might be like is something everyone does, and it doesn't hurt current you (unless you get swept up in some unreal fantasy, but hopefully you're more reasonable than that!).
6. "Damn, Channing Tatum is one hot piece of ass."
Just because you think Channing Tatum (or anyone else) is attractive, it doesn't mean it's detrimental to your relationship. What's detrimental to your relationship is thinking another person is "better" than your significant other, or acting on feelings of attraction in any way that goes outside the limits you and your partner have established in your relationship. But simply thinking someone else is hot is normal. I mean, other people are hot! You can't fight city hall! Just as long as you continue to treat your SO as special, make them feel special, and don't dally elsewhere, it's all good. I also personally think that while it's fine to think someone else is hot, it's not okay to gloat: too many times I've heard people in relationships get really graphic about finding someone else attractive, so have the common sense and decency to know what's respectful to your partner and what's not.
7. "Omg their ex [insert random worry here]."
It's not healthy to obsess over someone's ex, but it's perfectly natural to have feelings of curiosity, jealous or resentment from time to time. Don't feel guilty about it: everyone worries about filling someone else's shoes, what matters is how you deal with those thoughts.