I am so sorry that you are dealing with this, grief is hard at the best of times let alone when it is a close family member, I have not lost a parent but I can imagine it is horrific. I am so proud of you for reaching out here, to get some comfort and some support.
Grief is really hard as there are no rules, there is no handbook and there is no right or wrong way to do it, this is what sometimes makes us feel so lost on our grieving path.
I can hear that this new phase for your family with your mother's boyfriend and his son moving in is something new for you and your family, I am sure that this will take some time to get used to, let alone dealing with the grief and possibly the guilt that you are feeling for your dad as your mother has a new partner and you may be feeling like you are being disloyal to him.
How would you feel about letting yourself grieve? I think that you are holding it back and not truly letting yourself morn and feel the loss of your father..do you think this to be true? Perhaps starting with a conversation with your mother and letting her know how you are feeling, that you would like to be able to talk about you dad and let her know how you are feeling. This too might invite her to open up and talk too. I think if you start the conversation perhaps your brother and your mum may too.
There is no shame in crying, this is your father and you loved him so much and continue to do so, you are allowed to cry Elaine26, you are allowed to have bad days and to miss him, you are also allowed to laugh and remember the good times too.
I have found that talking here, being around others who are struggling has helped me so very much, I also found keeping a journal to be so very helpful, I wrote words that didn't make sense that were hurtful that were loving, all sorts of things depending on how I was feeling and what was going on. The thing is it is just for me to see, no one else so you can be as raw and honest as you need to...it helps so much to get it out.
I also need you to know something Elaine26..this was not your fault, it was not about you and it was not about your family. Suicide is mostly not about external factors in one's life and more about the internal sadness and pain taking over. Your dad would have loved you so very very much and would be very proud of you.
Please let yourself grieve, your dad will always be with you and have a special place in your heart.
Be kind to you