my mom isnt taking it seriously. i think she thinks its a phase or that im not serious or whatever

Your mother is an adult who is allowed to have her own thoughts and feelings. Please don't pressure her to change her opinions. Let her be.

Adulthood is new to you but this is what adulthood consists of: learning that you do not need your parents' permission, or approval, or even their understanding to do whatever you want to do.

This is not at all confrontational. I'm not asking you to shout at your mom that you don't care what she thinks! Like. That is a 15 yr old child's way of fighting for the freedom to do what they like.

Adults, in contrast, know that they are already free. They don't feel stressed or upset when their parents don't understand their choices or approve of their choices. Adults don't even ask their parents for understanding or approval, they just go about their business and allow their parents to react however they please.

If the parent of an adult on their own expresses condescending or disparaging attitudes, such as implying that the adult's relationship is frivolous, the the adult might say, "Hey, that's rude!" or roll their eyes or, if the disrespect from the parent is malicious, then they might say, "That's hurtful," and stop sharing details of their dating life with that parent.

When the parent of an adult on their own expresses confusion, like, "What does dating even mean?" then the adult's response is generally calm, relaxed, and curious about where their parents might be stuck. The adult might say something like, "Dating is a way to find a romantic partner, either for short term or long term relationships."

Crucially, the adult never tries to cajole or pressure or harangue their parent to give permission or express approval for the adult to continue dating. The adult is secure in the knowledge that they are adults, and this security allows the adult to give their parents room to feel whatever they want to feel, to let them work things out on their own.

So stop hassling your mom. Stop worrying about convincing other people to take your life seriously. Only you and your partner need to take dating seriously for it to be serious for you.