Get a book.
"What To Expect The Toddler Years."
Or take a parenting class or child development class.
Bear in mind, that even the most seasoned Moms, can go crazy with Toddlerhood.
BUT each year of a child, they go through normal age-related phases.
They are just a kid.
We are the "adult."
Hence, even if they seem difficult, they are just a kid.
Of course, a kid may "reject" you and be territorial. Their Mom, has changed. She is with you.
A child this age, DOES NOT even have, "emotions" fully developed yet. Nor the communication skills nor the vocabulary nor the ability... to understand abstract feelings. Nor do they even have the ability, to succinctly tell you, "how" they feel. Hence, they act it out.
This is normal.
She is a child.
Her parents are no longer together.
This affects... the child.
It can cause, deep emotional problems in a child.
She doesn't know, what you are.
or why you are around.
She is only, 2 years old.
They don't even know, what adult relationships are.
You need to keep this in mind.
And the Mom, should be nurturing her child, not you.
I don't mean that in a snarky way. BUT... if the relationship causes much emotional problems and behavioral problems in her child, the Mom... has to address this and make her child, the priority. Even getting child Counseling/Therapy if needed. Remember, the child's parents, are no longer together. Her Dad is not around. She... is confused. To say the least.
The "no" and "mine" phase... is NORMAL. Even with kids that have an intact family and 2 parents.
So, bear that in mind.
It is a hard age.
But, 3 and 4 years old... is even HARDER.
Your girlfriend's daughter... is going... through... a major... life... change. Her parents broke up. The Mom is with you. You live together. A child this young does not understand... all of that.
And what if you and the Mom break up one day?
That will also, affect the child.
At this young age, the child's well being, has to be, foremost.
They do not have deductive or inductive reasoning even developed yet. At this age. For example.
And most of all, do not "compete" with the 2 year old.
You are the adult.
Try to understand, how it feels for the 2 year old.
Her parents, are no longer her world.
The Mom is with you now.
That is a sudden... change. A big change in her life.
And a child either can or cannot, adjust to that.
Not even some teenagers, adjust or like, that their parent has a girlfriend or boyfriend.
You cannot have this relationship, only thinking of the Mom.
She has a kid. And the child needs lots of nurturing.
And, attention.
**And remember: Many adults... don't even know what their relationship is, nor can they define it or know what it is all about or if it is the one. Many adults don't even know the other person nor how each other feels. And a child doesn't even know, what "living together" means. Even couples don't know that themselves. So how can... a 2 year old, deal with all of this?
AND most importantly... a child, should NEVER have to feel... that they have to choose, between their Mom or someone else. If for example, the Mom is not pleased with her daughter being this way... then the daughter will be sorely, wronged. And a child, should not have to feel that they have to just please the boyfriend or else or that they are 2nd, to the Boyfriend. Or that if the Boyfriend is mad about the child, that the Girlfriend has to choose her boyfriend over her own child, in order to keep, him. That is not, a construct a child should have to live. It is dysfunctional.
You are not the child's parent.
You are not in charge... of disciplining the child.