This is one of the situations where it's vital to prioritize. You may not be able to stay close with all the people you would like to. If you only have a little time and energy to spend on socializing, you should choose a handful of relationships that you really want to last for your lifetime, and place those first on your list when you have time and energy.
A good friendship needs time and tender care, just like a seedling plant. If you care for it it will grow stronger and deeper roots. With care and dedication it will become a strong shelter for tough times, it may offer you a restful and rejuvenating place... and even some yummy fruit:)