It's not going to be easy, I can tell you that! Going from juniors to seniors is tough. But I'm not going to say that it is impossible either - not without seeing you play, anyway.
If you really love the game, I would like to encourage you to practice as well as you can, preferably under the guidance of an accomplished tennis coach. Your main goal should be to realize your own full potential, and not to become an ATP player, just for the sake of it. Your goal should be to be such an awesome player that it goes without saying that you'll be an ATP player.
If you feel like you just HAVE to step on the court and practice, then you have at least one characteristic of a real tennis champion.
Loving the battle (the match) is another one. Great champions love to play strong opponents.
Final point: if you always practice and play hard, and give it your all, both in practice and in matches, and you KNOW it, any mental weakness will be much easier to handle.