How will depression affect me over the long term?

Depression. Some people show a strong correlation between depression and heart disease. Make sure your cardiac health is intact by having your (fasting) cholesterol measured by as well as an appropriate examination by either a cardiologist or an internist-cardiologist every so often.
Bad if not treated. It depends on the presence of stressor/s, the kind of treatment that you're getting, your support system (such as support from family members and friends) and so forth. It is very treatable. However, without adequate treatment, it puts you at risk to develop cardiovascular problems.