Brother I'm sorry that you had to find us but glad you did. Based on what your WW has been cheating your for at least 4 years, it seems you only know the proverbial "tip of the iceberg". Men don't just typically stay in a relationship for years and send/exchange naked pictures to just "talk about the weather" when they meet in person, they typically have sex and lots of it, especially when they are already sexting with AP, btw I've read a lot of bs and ridiculous thing here on SI and other forums but her claim that she was "searching for ways to send him naked selfies" (as in all you have to do is literally take the naked selfie and reply to his) has to be a good contender to the top of the chart when it comes to just plain stupid things to say. Like others mentioned you offered R too quick, at this point you're not even sure of what you're forgiving, I always suggest D when it comes to LTAs like in your case, but if you insist on R here are some of the basic steps that have stood the test of time and could help you improve your chances a successful R:
1) Full Exposure: Expose the A with OBS (Other betrayed Spouse/GF), nothing kills an A faster than full exposure with OBS close relatives and close mutual friends WITHOUT warning (very important), exposure typically kills the "beautiful, romantic and special" aspects of an A and replaces them with pure shame and embarrassment, the more she hates the A, typically the more she will have AP who once exposed will most likely dump her in a desperate effort to save his own M, you will likely get a second set of eyes (OBS) to help with NC, exposure could also help with the remorse process, I would have her apologize to both sets of parents (if alive) for her huge betrayal.
2) Demand she gets tested for STDs/STIs (you should too): Again people involved an A when in close proximity typically have sex (why not?), some potentially serious deseases could remain dormant for years and early detection could help with treatment, also the "walk of shame" to the doctor's office could also help with remorse and drive home the potential risks involved.
3) NC Forever: Demand she sends an NC (No Contact) text to AP in front of you, watch her hit "send", short and to the point, no sweet goodbyes, then block him (including emails, social media, etc.).
4) Contact a D attorney to know your legal options, some may even do a free consultation, while you're at it ask about a postnuptial agreement, in case you later decide this was/is a dealbreaker for you and/or she cheats again in the future, remember she's now a proven cheater an liar.
5) She needs to get IC to find out her "whys", forget MC for now, at this point MC is likely a waste of time and money, the M didn't cheat, she did and continued to do it for years.
6) Full Access: She needs to offer full access to her phone and all electronic devices and passwords, don't fall for the "it's my privacy" bs, when married it becomes "our privacy", transparency is a must and non-negotiable.
If she refuses any of the above just file for D and have her served without warning, if D papers and full exposure don't shock her back to reality, then nothing will, if so just let the D process run its course and get out of infidelity, others will chime in with more advice, the collective wisdom of SI could help you go through your difficult situation, just look at your member number, we've literally seen this play out THOUSANDS of times here on ther forums, every case is different but cheaters typically follow a similar script, keep posting frequently.