Coming at this with no idea what adoption involves, not having done it, but having got pregnant accidentally at 19... my thoughts...
I accidentally got pregnant - was most definitely not ready. Adoption didn't cross my mind/no one suggested. I had an abortion. No regrets, but now (mid 40's) as a mother to one DS(8) I often think 'what if?'. That's it though. No real grief/regrets.
Ainsley, are you an only child? You seem far more mature than I was at your age. Your parents are still very young (lots of people their age are still getting/trying to get pregnant). Perhaps they would have loved another child? With their maturity they know that they will love that baby already - because it is yours. If I was your parent, in this situation, I couldn't bear to part with my daughters baby either.
Personally, I see huge benefits in your parents looking after your baby. As long as it's done on your terms (they look after him/her completely forever, or until you are ready/want to do so). As long as baby knows who's who, babies just need lots of consistent love and care.
A stranger adopting him/her won't remove the situation forever, one day that baby will come back looking for you, wanting to meet you and their real family and wonder why he/she was adopted. What will you say?
I am in no way judging, but at 15 you are so very young, if you were 20 and pregnant, but still didn't want to be, you might make a completely different decision, or of course, you might not.
By the way, I think you are very brave to even think about having a baby at your age (I opted for the easier option - in my opinion), but I don't think I could have given a baby away with such a perfect family solution. I think you may feel completely different when you give birth.
None of this is meant to offend or hurt. Just my thoughts as an older person. Good luck xxx