Because of your last situation, you may be slightly traumatized and not really know it. In order for successful penetration during intercourse, you must be well lubricated, either naturally or with the help of a store bought lubricant, i.e. KY Gel or Astro Glide. Your partner will never be able to penetrate you if you are to dry and not relaxed. When you relax, you are able to accept his penis without complication but you should be totally turned on, this promotes your own natural lubricant to kick in making penetration smooth and easy.
Penis's come in all sizes and women are built to take whatever they have to offer. So with some relaxation and some slippin and slidin, you should eventually be able to take him with comfort and ease. Have some lubricant on hand so you can apply it if you have to, in the meantime relax and allow yourself to be totally swept away.