Sex is the ultimate prize from a girl, and seeing you provide your "prize" from the get-go tells these guys that "you're easy". This is a bit long (didn't mean to), but here is what you can do to stop sleeping around and not feel like nobody values you:
Understand what you really want in life. This is not based on a relationship, but what is it you want to do when it comes time to graduate. This purpose will ultimately affect your behavior because if it becomes ingrained into your mind, it will drive your actions. For me, I always wanted to help people. So I decided to become a doctor even though the loans/interest are immense and hours are insane. To me, I had a vision of becoming a doctor, and that gave me self-confidence and a long-term goal. Whenever I feel a lack of self-confidence, I go back to my "higher purpose" and it validates me in what I truly want.
Write down your hobbies. Partying is not a hobby (in my opinion). It's just a form of fun once in awhile. A hobby is something you immensely enjoy and have high pride for. For me, it's reading and hiking. I love reading to learn. The first time I climbed a mountain and saw the sunset, I realized that we are very insignificant in this world. I've also met a lot of interesting people who have a lot of life experiences. Listening to them put a lot of my issues into perspective.
Exercise. If you start to get fit, it will make you more attractive and it will help improve your confidence. Additionally, it is also a great way to improve your health.
Make friends. Make friends for the sake of making friends, not because you need some sort of approval from them. When you have hobbies, you meet people with similar interests. Think of them as someone you can relate with, not have sex with.
Before graduation, find a job. Without a job, you are basically useless to society. You are dependent on someone else and you have no independence. Therefore, making money is important and becoming financially stable is the MOST important thing in life. Money drives everything we do: we pay rent, we buy food, we put gas in the car, etc. Don't take money lightly as something that's not important. It's very important, so if you do step 1, you'll be just fine.
When you put those 5 points into motion, you will eventually meet the same people with same goals, hobbies, and intent. It will also improve your self-confidence as an individual because you are worth something and people respect that. It will also help you understand that you need approval from others through sex to validate you as a person.