Brother!
Good for you for setting your heart on chastity

I am 19, and have been dating a GORGEOUS girl for 2 1/2 years. I know the struggles, especially as a former masturbation addict (fortunately I was able to kick the habit before we started dating).
The best advice about how to resist temptation is to resist as little as possible. That is, do everything you can so that you are not even tempted in the first place. If sitting with her a certain way puts thoughts in your head, sit differently. If kissing her a certain way arouses you, don’t do it. And I’m sorry brother, this may be painful for you to hear, but french kissing is an explicitly erotic kind of kissing. It’s really just an invitation for things to take a turn for the worse, and as a fellow Catholic looking out for you, I gotta say to steer clear of it.
Also, just like it takes two to tango, likewise it takes two
not to tango. If you want your relationship to be chaste, both you and your girlfriend have to be on board.
Again, congratulations and blessings on you for striving for purity! If faithful, you will have great rewards in this life and even greater in the next
EDIT: Also, do noooot marry because you feel like you “can’t wait any longer.” Again, I know how you feel, but that is simply no foundation for a marriage. That’s why chastity is so important (not just of the body but of the
mind) so that you can clearly discern your suitability for each other without being clouded by lust and other emotions. That said, your mind can still be clouded if you entertain lust, so have zero tolerance for it!