Dealing with a drug addiction in a relationship takes a lot of commitment from the party that isn't into drugs. You'll have to guard your finances, heart, health and attitude. Only you can determine if the relationship is worth keeping.
Focus on yourself (after all, the addict is focusing on himself, not you) and ensure that you're always safe, and that you have some funds should an emergency arise. Take care of yourself and focus on your needs and those of any children that may be involved as well.
You can encourage the addict to seek help without being judgmental. You can agree to go with them to counseling appointments if it would make it easier for them. It's all up to you and how much you're wiling to invest in the relationship.
It may help you to attend some al-anon meetings as well. Here you'll be with others who may be dealing with the same issues. You don't have to share details of your specific story unless you feel comfortable doing so.
Remember that this is the addicts choice, not yours. You're not responsible for their choice. You are responsible to yourself and your choices. You can choose to remain and live with the situation if everything else in the relationship is good and working well, or you can remove yourself from the situation and move on with your life. It's your choice.
For more information you can go to the resources below and read further.