You can put all the blame on yourself if you want to but you know that it was going to end up the same way weather you told your friend those truths or not. Maybe it sped the process up but it was better to happen sooner then later when things would hurt even more.
The only lesson I think you should derive from this is that you as a friend can offer opinion and comfort but in the end let friends follow their own road. I don't see how anything else is your doing if they where meant to be then things would have worked out the first time maybe the second time but seriously if they couldn't get their act together those times then they where right to take the hint and should deal with the consequences and the recovery process.
The only lesson I think you should derive from this is that you as a friend can offer opinion and comfort but in the end let friends follow their own road. I don't see how anything else is your doing if they where meant to be then things would have worked out the first time maybe the second time but seriously if they couldn't get their act together those times then they where right to take the hint and should deal with the consequences and the recovery process.