how can I accept my mum's new partner?

I am 19 and lost my dad 5 years ago. My mum has had one partner since but that didn’t come to anything. Now she has a new partner who she is very serious about and I am struggling to accept him. I feel he has no place in the house when that place is for my dad. It’s causing issues with mine and my mum's relationship and I am struggling to accept her partner. My brother has been more accepting and i'm worried that I am the only issue. It’s starting to cause issues with my mum and her partner and I don’t want that for her.
Asked by Bridget
I would first like to start with validating all of the emotions that you are experiencing. When we have lost a parent the flood of things that we feel and experience is huge and heavy and comes with the need for us to have time to process through it all and really come to terms with new dynamics that will naturally occur within our family system. It is imperative that we remember that what we feel is valid and reasonable, but what has to be looked at and addressed is how we allow those feelings to manifest in our lives. We want to keep hold of and feel all the things that come, without projecting that onto the other people in our lives, who are also feeling and grieving in their own unique ways. While you lost a dad, your mother lost a husband and her own grieving process is not going to progress the same. You lost someone in a role that is never meant to be replaced, and I really hate to pick that word as it is not even replacing but allowing ourselves to move forward. Your mom is likely in a place of forward movement where she can experience love and companionship again, and while it will never 'replace' your dad in any way, it is allowing her to continue to find happiness within her own life. 
For this particular situation that idea of replacement is key to look at. This person cannot and will not ever replace your dad, they are coming into your family in a new and unique role that is specific to that person. Sometimes it can be beneficial to work to take the time to get to know this person on a more personal level, versus knowing them only as someone your parent is romantically engaged with. When we can see them on a personal level and be able to learn who they are, likes, dislikes and find some kind of common ground, then it is easier to interact. This person likely makes your mother very happy, if she is allowing them in your life and so making sure to keep that piece in mind as well. She has experienced a significant loss also, but she also is in a place to deserve to find ways to keep moving forward with her own life and be happy. This does not in any way mean she is forgetting your dad, but that she is continuing to live with his memory while not feeling stuck in a place of grief. 
It is important to also remember that this person is dating your mom, so naturally they all will want to have a full relationship with you also, but that it is something that happens only within your comfort level. This person does not have to take on a specific role in your life, there are no rules or requirements there. You are able to have clear boundaries that things that are too close to a 'dad' role are off limits for you as you are not in a place where that is something that is comfortable or appropriate. When we can keep that bit of control over how the interactions exist and the closeness to ourselves that we allow then we can feel a bit less elevated by the situation as a whole. 
This process will likely have its ups and downs, but working to just communicate with your family the concerns you have and also be open minded that this person again is creating a new space, not trying to fill one that will forever belong to your father. I wish you all the best along this transition.