You're not exactly young. You're eligible to vote; you're 24 na nga. What am I trying to say? You're a Big Girl na; put on your Big Girl panties.
Que in vitro yan, a 1 night stand, a protracted affair, a legit/bastard -- even an incestuous coupling --- there is that 15yo your BF has to all intents & purposes acted as a responsible father to, all these years. They are father & son, till death do them part -- you just gotta suck it up.
What? Are you just being selfish? Yup, plus immature. I'd say that to my face in the mirror if I were in your shoes. Misplaced your Big Girl panties?
To love truly is to love not just his warts and all, but also the people he loves.
Who said loving was easy? You love even when he is/acts unlovable; same with the people he loves.
Setting aside your trust issue (he did not reveal this 15yo till much later, you say) -- let's focus on your behavioral issue vis-vis the teen........yan tanong mo di ba .....
Pwes, ....
Act like the woman he sees you as -- one capable of making a life with an older, more experienced, established man.
(Hindi naman siguro yan as young as you, kasi may 15yo, kahit na ba in vitro. Usually nga, pag in vitro eh -- after all effing attempts, medj matanda na. But don't mind my imagination, what do I know anyway.).
IF you are convinced this 1 is a keeper, that he will commit to you -- eh, a Big Girl will get into this relationship with a "man already with an accessory" with depth/maturity, and style/poise.
Don't be like a needy child, needing to be assured with PDAs. Be confident that you are loved. There are more significant indications of love. (It is flirting that needs all these overt acts of affection/affectation.).
If you are convinced this love is worth your time, STEP UP to the challenge.
Sure, you can handle that 15yo. -- research the ways, talk to teen experts.
Don't use your 24 years as a cop-out excuse.
If you can't handle a 15yo, what else can't you handle? Kayang kaya mo yan! Just focus, be patient, open your loving heart.
Time to put on the Big Girl panties.