The measure of success is going to be different for everyone - different goals, different view of what their personal success is going to be, what kind of timeline they plan to hold themselves to, etc.
I think one of the few ways to truly measure a failure would be someone that never takes action - someone who sat idle and didn't make their situation better. If you do end up going bankrupt, of course that seems like the worst situation you could put yourself in as an investor. However, you will have learned some incredible lessons from the mistakes made, albeit costly ones.
One of the best pieces of advice I've received on the topic of real estate investing came from a gentleman who had the same advice passed on to him from an experienced investor. Approach your investment portfolio one property at a time. Don't look too far ahead - just focus on that next deal. Take care of all the details for the next property you plan to acquire, make sure it's rock solid, close on it and then determine what your next steps will be. If you try doing too much at once, this is when you get swamped and the probability of making mistakes increases.