How do you know that you are really done having children?

I have a two yr old son and a 1yr old daughter (15 months apart). It's so difficult having babies so close together, and I've had lots of back/pelvic pain since I had my first baby. I'm not sure if it will get better. I cannot be on birth control, I've tried everything and I get every side effect, plus I have depression and it makes it way worse. So we're using condoms, which sucks. We've talked about him getting a vasectomy but we can't come to a decision. We're willing to adopt, but don't know much about it. I don't want to make a decision I'll later regret.