Feeling insecure in a relationship is quite common.
First of all, maybe you have a very good reason to feel insecure. Maybe you can't trust your partner. Maybe he rejected you before, maybe he broke up with you before and you left worried about your future now. It's a very good reason to feel insecure with someone that you can't trust. It's nothing wrong with your feelings, it's that you are with a wrong person.
Rule number one to feel secure in a relationship is to find a proper partner. Find someone who needs you and who you can trust. Trust your gut feelings. If you feel it's not right, it's probably not. You should know what you want in a romantic partner, and if he/she constantly rejecting you, then yes, you better be worried and insecure!
Start with a new partner who wants to do anything he can to please you, find someone who loves you and enjoys you. Yes, it's selfish to like someone who doesn't like you back, it's selfish that only you enjoying the companionship. So, find someone who would want always want to be with you.
Simple requirements to feel secure
Don't trust anyone in the beginning of the relationship. People change their minds if they want to be with you and don't be with someone who rejected you. Look for signs of rejection and acceptance. If a person doesn't want to be with you, let it go. Find someone who will love you for who you are.
If you are still with someone who rejected you before, definitely, you would feel very insecure. You would always be affraid what to say what to do, but no matter what you do, you can't make someone to like you. No matter how nice you are, no matter what kind things you do for that person, nothing can make him/her to like you or need you.
Everyone has specific needs and if your needs are very different from your partner's needs, it would be very hard to enjoy being together. It could be difference in personality types or it could be other differences. The lack of desire being in a relationship of one of the partners is a major reason to feel insecure.