The premise of Law of Attraction is actually simple. But, to reach this simplicity, you often have to travel through "layers." Self forgiveness is one of those layers; and it influences use of Law and success results.
Do you ever wish you'd done something in the past differently, or think about how results of one or more mistakes caused you to feel pretty bad about yourself, or humiliated? Or how maybe you allowed things you wish you hadn't. Maybe you had some rough patches in life and feel you could have handled such times better. Maybe... But, those times are over. It's what you're telling yourself now, in the present, about those past events that matters.
Ernest Holmes wrote what follows in all capital letters, but he wasn't part of the electronic age where that would be considered rude; so just imagine this quoted part in all caps for emphasis: "No person whose entire time is spent in the contemplation of limitation can demonstrate freedom from such limitation!" I would add that you don't have to do this the entire time for it to affect you, just enough of the time and with enough feeling. However, Holmes' point is well taken.
There are many things we might need to forgive ourselves for, including thinking less of ourselves, believing we don't deserve better, criticizing ourselves for not being perfect--whatever THAT is, or holding our past against us in some way. These are forms of non-forgiveness we practice on ourselves all too often, without realizing the full impact.
Carrying self-condemnation and condemnation of others is truly a burden. Because of how the Law of Attraction works, we add to the weight of this by holding on to such thoughts, which multiplies their effect on us and our lives. This is a form of resistance... resistance to well-being for us and others. It resists the feeling of freedom we want and pursue.
Abraham-Hicks wrote, "If you are resisting anything, you are focused upon it, pushing against it, and activating the vibration of it--and therefore attracting that which is like it." You might ask yourself in which ways you resist success in any area of your life--work, relationships, finances, harmony, serenity, health, etc.--with condemning or self-condemning thoughts. This can be a pretty revealing thing to do.
There are lots of writings about forgiveness of others you can access, so I'll leave that to you, if that speaks to you at this time. It does influence your success-ability, though. But, I focus on self-forgiveness here because it's the one we carry closest to us.
Forgiveness is really about putting what's been an emotional burden to us down, about letting the story go and never repeating it again--and telling a new, better story or telling the old story in a new, better way, if telling it is truly beneficial to increase wisdom or do good in some way. The burden is all the thoughts and emotions you have about something that happened or what someone or you did, which you drag like boulders behind you through life. When you replay something in your mind, you relive it in your cells and glue it more firmly into what you perceive as "truth." Positive or negative. And the Law of Attraction is bound by its nature to supply you with whatever it takes to match "truth," as you perceive it.
Perhaps you pull up memories of a relative telling you something negative about you, or your third grade teacher saying you weren't smart--all just their opinions, of course. Perhaps you've made this your practice, without realizing what this does to you. When triggered emotionally, you may ask, "What happened in the past?"--which pulls up negative memories instead of asking, "What can I tell myself that's supportive and moves me in the direction of my desire... makes my feeling-vibration a greater match with my desire?"
You can't imagine of yourself as More if you see yourself as Less. Any negative self thoughts are a form of resistance to the truth of who you really are. One way to practice self-forgiveness is to stop allowing the opinions of others (past or present), or what you imagine their opinions are, to have more weight or merit or validity than what Source thinks. Source loves you, unconditionally, because you and It are one. It may be time to create a new concept of or for your Self, one that lets you see how much good there is about and in you. And let this new concept be the one you wake to each day.
The burden of any self-condemnation blocks success in any area that you desire to improve or create. The lightness of release allows you to more easily focus on creativity and desires and results than on condemnation and how bad such thoughts make you feel. Such thoughts cause you to resist success instead of allow it--because they cause you to believe you don't deserve better.
Deserving is a man-made concept. Source doesn't think this way, because you are an expression of IT, and all you are IS Source--there is no separation except in your thinking. Otherwise, Source would have to determine Itself as undeserving at times, as well, and that can and will never happen. Nor does the Law of Attraction take deserving into consideration either. It's an impartial, non-thinking "mechanism" that responds only to what you think/feel into it.
The "lighter" you are from release of self-condemnation the happier you'll be. The happier you feel the more "attractive" of what you desire you'll be. Stop feeding self-condemning thoughts and you can begin to believe success is as much your right as it is anyone's--and it is! You can observe how you feel, to know whether your energy vibrates as a match with what you desire (manifestation) or matches its absence (resists manifestation). It's far too easy to tell yourself negative stories about you--and worse!--to believe them. "A belief is only a practiced vibration." (Abraham-Hicks)
Sometimes negative thoughts about the self happen so quickly, you notice them but not for what they are, much less their overall effect on you. This means a lot of practice happens with such thoughts. Begin to connect what you think and feel to what you experience and you're on your way to understanding the Law of Attraction and using it more in your favor, until you become so good at this, it becomes automatic to you.
Practice makes progress.