These tip should be effective at resolving your, ah, issue, anonymous: http://bills-pokedex.tumblr.com/search/separation
Of course, it’s important to be patient while training your bulbasaur (according to the tips above). He is still a child, but with enough patience and care, you should be able to make the experience of being weaned off your constant presence a little less traumatic.
Best of luck!
The Pokédex According to Bill 