Swimming - I am so sorry that you are hurting. Truly.
You are not "hard" on yourself. You have some inappropriate guilt. The affair, sister, is his choice. He is an adult whom you have NO CONTROL over, so cut that thinking out right now about how you are responsible for it.
"he needs to do things to make me feel secure" - again, you are suggesting that one grown up should be responsible for another grown up's feelings. Stand on your own. Feel what you feel and take responsibility for that alone.
Forgiving someone does not mean you put the offense "behind you". It means you take your hands off their throat for doing it. Whether you forgive him or not, you will have to deal with the hurt, the wound, the scar of what's been done. The forgiveness is as much a gift you give yourself as anyone else.
Greek
Me45 H46 T25 M22 S21 & 19 D13 Separated and filed 8/08 Moved home 11/08