You should give up. Why on earth would you want to marry a man that makes you feel that you give everything and get nothing back. You would be wise to understand where your tendency to fall into this pattern stems from. Does this recreate an experience from your formative years? As for this man. He obviously has mixed feelings about intimacy and commitment, and marrying him won't make this problem go away.
You are a sweet woman and very willing to put your needs aside to be there for him. But what about your needs? This man withdraws for long stretches, and the problem isn't going to go away by itself. What can you do? A couple of choices: You can ask him to start talking with you about his mixed feelings rather than acting them out. He needs to understand that when he goes into action (withdrawing for 3 days) he is damaging the relationship.
If he wants a relationship with you, he must talk about his fears whenever they arise, not act. Every time he feels nervous, he is to stay in contact (not pull away). If he can do this, you have a man that 's workable and a relationship that has promise. If he can't control his behavior and must withdraw, you are in for big-time misery with this man. And, you really need to ask yourself if you want to marry a man that will be abandoning you like this. Another option is to mirror his doubts and tell him that you aren't ready to get engaged. This technique will probably bring him closer, but it won't actually heal his underlying fears and issues.
No matter which approach you choose, I wouldn't be marrying so fast. You need to find out whether this man is actually capable of a relationship before commiting yourself for the long haul. My best wishes to you. You are lovely, generous woman who deserves to receive back as much love as she gives.