Get an android phone that you can have two accounts on (unless she's tech savvy she won't know about this and it isn't obvious). use an ecrypted email on the second user. Delete everything every time you log off. Use TOR for anything internet related using a search engine like duckduckgo on it. Nothing stored. Use a disposable text app as well. Delete everything on it every time as well. Never log on to second user in her presence.
You've now eliminated almost everything she could find on your phone and if she finds the second user (not likely) then just claim you were playing around with it. She won't find anything even if she looks if you follow above. This all takes a little bit of effort but once you do it for a while it is second nature.
If you can't do all of this then get a burner phone and store it somewhere other than your house and don't use it at home. This is a bit more difficult but if you aren't at all tech savvy then this would be the way to go.