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

Take a deep breath, exhale slowly. Repeat. Realize that it is just a moment of weakness. It is not real. It is not you. Just a moment that will pass and things will get back to normal again.

First of all, the fact that you know those are irrational feelings is a good one: it means you are still aware of very important nuances and it's the first step to fight them. You recognize them, so you can target them with your efforts. How to stop them? Focus, first of all, on changing them. They probably steam from experiences or situations that trigger something in you, something irrational. First of all, spot those situations, and then ask yourself: how could I react rationally to it? Doing this for long enough will replace the automatic irrational thought/feeling with the rational one, and it may lead you to a point where your brain doesn't fall under its influence anymore. It will take time, patience and perseverance, though, and in those I wish you the best of each! You can do it, you are worth the change.

When you know the feeling is irrational it its definitely a negative feeling. Instead of seeing the easy negative try to search for the positive in the situation. Eventually your mind becomes trained to automatically see the good in every situation and the negatives seem rather petty.

In my personal opinion, I believe that NO feeling is irrational. While it may make "no sense" to somebody else, that doesn't mean it is without sense. Your feelings are valid no matter what, and once you understand this, the sooner you'll be able to put your best foot forward and decide what the best option for you to take is in order to overcome any negative feelings.

When you are lost in a trance of feeling or thinking, and you recognise that it's not healthy or helping you, switching up your activity can help reset your mood. Do something different - even if it's as small as moving into a different room or putting on some music or calling a friend/relative for a chat. If you can find an activity to interrupt the compulsive feeling or thinking behaviour, you can take advantage of the space and 'pause' it brings you to make a more empowered choice about your mind's focus. Personally, I find exercise to be an excellent mood-shifter. It's hard to remain stuck in your mind when you are concentrating on pushing your physical abilities. And the fitness-related adrenaline and endorphin boost can help improve wellbeing. Being in nature can also be very healing... A change of scenery... Fresh air.... A chance to appreciate simple pleasures. You may need to return to whatever the 'thing' is that you are thinking or feeling... So that you can process it. Compulsive thoughts/feelings can be linked to real issues that do need your focus and attention... When you are ready.. But you don't need to be held prisoner by them. You can choose when and where to focus your attention. So become your mind's "manager" and interrupt the programming.

Perhaps try to get to the root of the emotion, for instance what caused this irrational emotion to appear, and then deal with it accordingly? Looking from another persons perspective may also be helpful :)

For me, there's an analogy that works: if you are dancing with lots of turn, you are told to "spot" something to keep your balance and focus. I think mental "clutter" can benefit from the same idea.

It all depends up on your will power to overcome such thoughts. It may be hard in beginning but with firm will, you can ignore it. Try to do things which make you feel happy or which are of priority and try to focus on one area at a time. Spend time more with those who love you and care for you, give yourself some space and time. Instead of doubting yourself or your potentials, try to manage with whatever you possesses, as you know its irrational, you can surely avoid it.

Your feelings matter! They are never something to be taken lightly. Look at your feelings and assess where they are coming from. Try and resolve these issues. If that is not doable, take time for yourself and find out what makes you happy. Do that and distract yourself for a bit in order to calm down. Try breathing exercises and check out 7Cups Anxiety Guide. You can get through this. You can do this!