She doesn't want to talk to me so often

Shadow says:
 I'm in relationship with a girl for 2.5 years. She is in her mid 30's (9 years older than me). We stay in different cities - about 4 hours drive. We used to talk on phone every night, meeting each other every weekend.

By time goes, meeting weekly has became biweekly and sometimes even lesser. She started to hang out with her friends a lot. Long trip (around 15 days), short trips(1-2 days) and hang out late frequently.We talk over the phone lesser, 3-4 times a week instead of everyday. She used to spend her semester break with me, but now, she prefer stay back with her friends.

I told her about my feeling of being ignored by her. She just take it easy, saying she needs some space. She told me that loving her doesn't mean to occupy her.

It's sad to hear that she said she's busy and don't have time to meet, but she has extra time to hang out with friends. That's truly hurt.

I just hope to spend time from her. For me, talking with her over the phone every night is the only time I could be with her as we're living apart. Am I asking too much? Besides, I doubted if she will spend more and more time with friends?

I even thought of giving up this relationship. What should I do? 

It's very difficult to keep a long-distance relationship going. It only works if the two people involved are really crazy about each other. I'm afraid that clearly your girlfriend likes you but is NOT besotted with you. The relationship is cooling and will eventually go completely cold. Your hassling her is only speeding up the end of the relationship.
Ladies do need their friendships and for women good friends are as close as sisters. With you not being around so much, she has clearly formed some close friendships. You could just accept a cooler relationship, but I think that's delaying the inevitable. Make the break now and look for a relationship that has the intensity you are looking for from someone you can actually get to see.