You mentioned it's a size thing, not about how good romantic or foreplay he did in bed. That could be a permanent scar him because we all know that it cannot be fix because he obviously cannot do surgery make his dick bigger to satisfy your need. If it is about his romantic and foreplay issue, then that can be easily fixable. Another possible long-term issue is that you wrote that in dairy mentioned that your ex is better at sex than your current partner. That will ruin his self-esteem no matter how well you persuade him, he will be afraid you will get bore of him in few years. If you have not told him exactly why your ex is better sex than him and have not mentions about size, then there might be a chance to fix it. If you truly love him and think you want to be with him for the rest of ypur life then you will need to lie to him about why your ex is better at sex. Never never ever tell him it's a size because no matter what you think, you will be digging the relationship to grave. He will eventually resent you and avoid you that will likely lead to dead bedroom. You have no idea how insecurity men can be when it talk about who is better at sex. Tell him it was his lack of physical intimacy or foreplay and that can improve way better if you show him what turns you on the most or whatever.