He wants another chance, but...

My partner left me a few months ago and has been struggling with depression since then. I am aware that he is in pain, but this was his decision and there was nothing I could do to save our relationship. A few weeks ago, he called and spoke to me about how he would like another chance, and I told him I would think about it. The reason I can’t commit to this is because I believe the problem lies with his mental health. Getting back won’t solve anything unless he prioritises fixing his personal issues. Until that happens, I feel as if this will only make him temporarily happy. I care about him and would love to give this another chance, but it will only lead to another possible break-up and cause me pain all over again. Am I being selfish for thinking this way? Should I get back with him and help him do this together? 
You don’t have to commit to a relationship to help someone you care about. If it is his mental health, he needs professional help, and a relationship is not the solution. You don’t have the tools to help him, and he may not have them either. You can always talk about possibilities and taking things slowly depending upon how he addresses this. If he wants you back, he has to show willingness to work on his mental health because putting yourself through this again is unfair to you. Talk about counselling, ask him to take steps towards getting better, and revisit this conversation a few months from now. This isn’t selfishness; it’s about helping in a more tangible manner and being pragmatic about what can happen when you are both in a better place.

How do I know if I am moving too fast in a relationship?
Talk about it as often as possible. There is no manual, so you and your partner are allowed to move at any pace as long as it is one you are comfortable with.