Divorce is hard on everyone, especially children who must accept that their parents are no longer together. Unfortunately, you cannot control your ex-husband. He doesn’t have to force your sons to spend time with you, but he can encourage them to do so.
Of course you miss your kids and want to spend time with them. The best thing you can do now is continue to call them and make attempts to see them. Remind them that you love them. Continue to show interest by sending cards and attending their sporting events. Try not to get upset with them or their father, as this will only cause more negative interactions between you.
You can also try talking to your ex-husband about encouraging the boys to see you more often. It sounds like your divorce is fairly recent, so it may take a little time to sort out the new family dynamic.
Should the present situation continue for long without any visits, you may have to seek legal help to gain rights for visits with your children.
When the time is right, you may want to consider beginning family therapy with you and your children to build a stronger relationship. It will also help you work through some of the negative emotions you are all experiencing.