Is there any way to gain fat in my Mons pubis?

Since you have identified this area as being deficient in volume fat grafting would be an excellent way to add volume and suddenly better than any alternative that I can think of.Fat grafting (transfer) is a very versatile technique that can be used almost anywhere to add volume using your own tissue, and the Mons pubis seems to be an excellent place to put it if that's what you need.

Fat transfer to your mons pubis is possible. However, not every patient is a candidate. Consult with 2 - 3 experienced and expert board certified plastic surgeons to understand your options.

Since you are very young, I believe fat transfer to the mons pubis is a very good option to plump the mons