First of all, there is nothing wrong with you. You weren’t born defective in this area. This is a pattern that you’ve learned and since you’ve learned it, you can change it. So, number one, let go of the idea that something is simply ‘wrong’ and embrace the fact this is behaviour that is entirely within your power now that you’ve noticed this about yourself.
Second of all, this isn’t some anomaly. Commitment issues of this type are very common. Often, they don’t come to the fore until later in life, so you’re ahead of the game if you’ve caught this at age seventeen.
Usually this is type of behaviour is the product of low self confidence in yourself or in the relationship arena. You know you want a relationship but some part of you doesn’t believe you can hold onto a good relationship even if you get one and on a subconscious level, you anticipate that it will all end in tears and you won’t be able to handle it. Of course, break ups do happen and people do get hurt in relationships, but just as often, relationships can and do work out beautifully. It’s a nothing ventured, nothing gained situation.
Apparently, you’re more risk and loss-averse than you are open to opportunity for success, so you shut down relationship opportunities before they have a chance to become losses. You only let yourself get pressured into situations because if you didn't get pressured, likely nothing would happen. The people who are pressuring you likely have the best intentions and want to see you succeed, but they are not you, so they don't know what *you* want. Stop being passive. Take control and have the courage to go after what *you* want instead of avoiding and allowing yourself to get strong-armed into situations that don't meet your standards.
You must embrace the fact that life is sometimes going to give you lemons and that even though you may get bruised and disappointed from time to time, that you and your heart are not in any kind of danger. People are way more resilient than they give themselves credit for. The truth is, we actually end up hurting ourselves more when we try to avoid getting hurt than simply accepting that pain is the other side of the coin for life’s joys.
Make peace with both the good and bad sides of getting into a relationship. Make peace with the fact that you are not as weak or fragile as you may think you are. Once you do, life will open up to you.
Bless.