How can I stop feelings I know are irrational but still take over my mental and emotional state?

Simple. Focus on your breathing and staying in the here and now. Even stare at something external if it jolts you enough to get out of your head. The challenge is to find what helps for you personally

Sometimes it can be helpful to take a minute, clear your mind. Take some deep breaths. Then when you are feeling better go over your feelings and try to address them and understand why you think that way and what the actual situation is. It is okay to have irrational feelings, but it is important to understand why you have them

if u have been in the same situation before , just remember how it ended and how u felt. if it is a new situation take very deep breaths and think if it is a good idea or a bad one and decide if u really want to do this .

You can't stop those feelings, but you can manage them in a healthy manner and take control of them rather than them taking over you.

Identify them - give them a name! When you are feeling anxious, and it seems irrational, remind yourself that the feelings just mean that "Anxious Joe" is back and he will be gone soon. Acknowledging the feeling can help you control it.

Relaxing and trying to focus on living in the moment. Also thinking about something else.

Take a moment to define your feelings and try to think back to what caused them. If you can identify the emotion and the cause, you're on a good path to rationalizing your mental state.

Personally, I find that reminding yourself that you're feelings may be irrational eventually helps to stop them from taking over your mental and emotional state. It takes time, a lot of things do. I also like to remind myself of how situations would play out if I acted on my irrational feelings. This usually helps me to realise i'm making the right decision to overcome my feelings and move of. Of course, if this will not help everybody.

Meditation is one of the key mental healing/physical actions you can do to balance feelings. Also, yoga, exercise and positive thinking can help. A old trick I like to use is to write down feelings that have been bothering me on a piece of paper, then tear the paper into tiny pieces to then throw it away. This is a physical representation of how I deal with ill thoughts, feelings and emotions.