I was in a relationship for 3 years. I love too hard. Is that a problem?

It sounds like you are struggling with grief over the loss of a relationship. Having a relationship end when you did not want it to can be so very hard and a lot like grieving the loss of a person. You not only lost the person being in your life, but you also are losing the companionship, love, support and the future you envisioned having with that person. Grief does not have a time clock. Each person heals and goes through the grief process differently. Give yourself time, love and compassion as you continue to grieve the ending of this relationship. Don't hold back the tears when they come. If you feel sad, it is okay to sit with your sad thoughts. They will pass eventually. Grief can come in waves but just as the waves hit the shore, they always return back to the sea. 

As far as your question goes about loving someone too hard, I do not think that is the problem here. What may have been the problem is that the two of you were not compatible. Maybe the two of you came into this relationship wanting and needing different things. Of course, it is hard to really know for sure with not knowing more about your specific situation. I hope that the outcome of the ending of this relationship will not discourage you from seeking out or being vulnerable in any future relationships. Meaning, just because this relationship ended this way, does not mean that the next person you allow yourself to have feelings for will end the same way. 

I also think it would be healthy and helpful for you to start focusing on yourself and building your own happiness before entering into another relationship. What are things that make you happy? Are there hobbies you want to pursue? Are there friendships you want to strengthen? What are your personal interest and what keeps you going and motivated and hopeful about your future? Perhaps focusing on gratitude and the good things you have going for you in your life will be a good start with working on building happiness for yourself.