In my family we say that as please don't be long, part of me always reacts as a I'll be as long as I like but the logical part of me knows this is silly and we only have one bathroom so you need to consider others in the house.
I don't know how I'd feel should my mum get a new partner and move them in. I'm in my early 20's and live at home when on holiday from uni. My dad died when I was a preteen and I know I'd not like someone else living there in his place with my mum in my house. The only home I've ever know. Yes it would be my mums decision and I don't want to deprive her that happiness but I'd feel annoyed and displaced a lot of the time, because the family dynamic would change and I only know the dynamic we have now.
In the past as a teenager she refrained from all relationships because I told her if she started dating anyone or brought them back to the house at all I'd move out and in with one of my sisters or I'd make her life so difficult she would throw me out, or I'd go into school and refuse to go home forcing them to call SS. That if I had kids she'd only ever see them away from them and the house and not be allowed to mention him to them as he wouldn't be family and that she'd never have a picture of them as he has no right to see them ever.
As an adult I realise what a hideous and vile thing to do, say threaten and person I was back then (I'd of been 12-15 and it was around that time I'd had a mental breakdown but that is no excuse) but my mum doesn't hold it against me because I was scared and hurting and the very mention of her dating was enough for me to have a panic attack. She understands somewhat though as even in her 60's she says she has no idea how she'd feel if her father ever remarried after her mothers death. So it's difficult whatever your age.
As an adult I can see anyone would be lucky to have her and no one will ever be good enough for her, and while I'd not like it. I would be supportive to any relationship she chooses to have, but that won't make it easy.
You have every right to feel the way you do and 'stake your claim' because you were there first. But you need to deal with this honestly and calmly, talk to your mum 1-1 and try getting to know your stepdad. If there are things you feel strongly about like family traditions, teach him about them so he fits into what you have instead of feeling like he's an interloper consider him as a family member in training and introduce him to the way you guys do things, with an open mind because somethings in your family will change too. That just the way it is and it sucks, but your mum is happy and that's what matters. You both love her so you already have common ground.