How do I deal with someone who constantly interrupts?
This really depends not only on the other person, but on your perspective of them. For example, are they A) doing this on purpose, or are they B) simply a clueless narcissist that has no idea they’re even doing it or that you’re bothered by it?
If it’s A, if you’re able to, try to convey how this makes you feel. Chances are, if it’s A, they may not really care. I suggest if you’re in a position in which you can distance yourself from them, do so.
If it’s B, definitely let them know that this upsets you. If they continue to do so and they don’t acknowledge or apologize for their behavior, then they belong to category A.
But there’s also category C, in which they do know they are doing it, but you also know for a fact that they’re not meaning to hurt your feelings and that they simply talk a lot and interject their thoughts and opinions. In which case, you can either try not to take it personally, or if it’s simply too overwhelming, try letting them go, or at least spending less time around them if you can.
The bottom line, really, is if it upsets you, get away from them if you can. The only question is in how to do so and to what degree, which depends on why/how they’re constantly interrupting you.