Everything you witnessed, experienced, and were taught as a child still affects you to this day. All of those experiences not only affect you personally, but they affect your relationship in either positive or negative ways.
Let's start with the positives! If you grew up in a home with two loving, supportive, respectful parents, you most likely will be loving, supportive and respectful towards your partner. You will treat them well, prioritize them, and work as a team. And if you were raised with love, support, and respect as well, then once again you will most likely treat your partner the same way. The best possible outcome! (short and sweet and to the point for this one).
Unfortunately, everyone grew up with parents who honored each other, or with parents who raised them properly. This may resonate with you, or your partner, and sadly that is why a lot of issues may be coming up in your relationship. If you experienced a parent or loved one leaving, then you may have major abandonment issues with your partner. You might fear that they will leave you too, so you obsessively cling on to them which actually ends up pushing them away. If your parents did not give you attention for doing great things, then you probably resorted to lashing out for some type of feedback. Guess what message that sent you? That lashing out was the only way to receive any attention and if that's the message you received, guess what you're going to do to your partner? Lash out at them at any moment. If your parents were constantly negative about everything, from the uncontrollable weather patterns to the color of the shirt that some random dude is wearing, you are naturally going to be negative about every little thing too. And if you're constantly negative all the time, you're most definitely going to push your partner away, especially if they just want to be positive.
Obviously there are a ton of other ways that our childhood affects our relationships. If you notice that you have any behavior that is negative affecting your relationship, you need to begin the process of changing them. Start by identifying exactly where those negative behaviors came from and learn to accept it. Once you have come to terms with your past, communicate with your partner and come up with a plan to get rid of those negative behaviors. Give them strategies to implement when they witness you repeating a behavior, and also take a mental note of everything you are doing and saying. If you feel the behavior coming on, remember your plan and stick to it. It will be hard, and it will take a very long time, but it's the best thing you can do to not only help your relationship, but to help yourself.