If you're looking to up your skills in a new language or just "try" the new activity, then it makes sense that you would quit a few months later: you've acquired what you wanted.
If you wanted to keep going and pursue that activity for longer, I think it's a matter of not making the activity conditional on it "satisfying" you because as someone mentioned in another comment, you'll lose that initial dopamine hit once the activity becomes familiar (or boring): so the solution imo is to actively decide that whether you enjoy it or not, you will do X. And that's just a matter of learning to commit to something.
I used to be all over the place and chase new ideas over existing projects all the time, but it wasn't until I made a decision that OK I'm going to stick with this no matter what (at least for a period of time) that I started finding success. Now I think consistency is 80% of success.
Another thing I find useful to keep going is to find a way to keep getting small "wins" every now and then in the journey: the small wins make you feel like the past time invested was worth it and make my brain eager to continue as it anticipates the next one.