Please don't feel guilty about masterbating or watching porn. These are often stages that people go through. Since you are not happy with the action anymore , change it. Do something else worthy, and AnnieBrooke had a great idea to do some type of volunteer work . if that is possible after you look after your education and work prospects.
You say you are 26 with no girlfriend, YET. (right? no gf, yet...) I think before you are personally ready to have a gf you might want to talk to a therapist and learn to let go the things that are making you crazy. You need to get to a calm place before you can fully give yourself to another. Can you get to a therapist?
Also, are your working, or going to school? Do you live by yourself, or with your folks? Do you have friends, (where you can meet girls)?
I wish you did not have such guilt laden onto the situation. It is normal to masturbate. No, it's not acceptable to spy on your sister and especially not to touch her, but the way to get past that past shame is to stop, which you did, and to apologize to your sister if she knows you did it. (Do not tell her that you did it if she does not know, since it would be distressing to her to learn of it and it would be unnecessarily unkind of you to hurt her. But if she does know you did it, give her an apology from the heart, and some serious promises that you will never do that sort of thing again to any woman.)
Then to feel better in your mind about yourself and to get your life more balanced, possibly do some altruistic things that make you feel like you are contributing to a good cause. Find a cause you care about, and volunteer with no thought of gain, and work very hard. It will help you realize that you are a worthy person, in a world that needs all the help it can get from worthy, hard-working people. You are not a loser until you give up. You can become a happier person if you are contributing to something that needs help. Good luck to you.