Hi, miamiblues, and welcome to Marriage Builders.
It's no wonder you feel devastated; your trust has been dealt some heavy blows.
Have you known about the affairs for a while and just recently learned about the premarital partners, or is all of this new knowledge to you? Either way, you're dealing with lies and betrayal, but the timeline does make a difference as to how you are perceiving things, and also how she's feeling.
How long have the two affairs been over?
When you say "you tried to get her to open up and tell you the problem but it took 20 years" -- are you saying that sex has been a problem for the entire 20 years of your marriage? And you have tried to get her to open up for the entire 20 years, but she only has done so recently? What did you do in the past to try to get her to open up, and how did she react? What changed recently - did you change your approach, did something happen in her life or your lives? Why do you think she opened up to you right now?
You say you love her and want to overcome this - then you should thank her for her honesty in telling you the truth. Tell her you know it must have been very difficult and frightening for her, and that you appreciate her efforts. Let her know it hurts you.
If you haven't read up on love busters, read about them now and avoid reacting to her confessions with any love busters. You have to be a safe place for her.
Regarding your inability to get past the constant fear, well, you should let go of that right now for a couple of reasons:
1. It is up to her to earn your trust; it's not up to you to "get over" the fear.
2. The fear will motivate you to do the hard work of improving the marriage so that you don't fall back into the old way of doing things. You will be motivated to build a better marriage - and it is HARD WORK. You'll need a lot of motivation.
Keep posting.