No idea what to do, and need some advice

Hi, I'm looking for some help and advice regarding my marriage. We've been married 8 years, but the last couple seem to of been a real struggle. My wife has suffered on and of with depression, due to the death of her parents. My wife started writing to prisoners in the united states, as she thought it would help with her depression, this was ok with me, although I found it a little odd, as anything to help with her depression can only be good, or so I thought. Since starting to write to these people, she has obviously built up a "friendship" or sorts with these people, and it now seems to have reached a point where she is more excited to recieve their letter, than see me when I come home, I realize this sounds incredibly self centered, but that's how it seems. A few months back I found out she had sent one of these people money to buy a phone, cue the argument, and me telling her I would prefer her not to do this again, however, I checked our bank last night to find she has sent more money to the same person. Now I also think she chats to various men on WhatsApp and various other apps. I'm pretty sure of this, as she will be on her phone a lot, yet if I use her phone for anything all the apps have had the conversations deleted. Now this is where things get awkward for me, as it's very hard to talk to her, as she gets VERY defensive/angry, and before I can really say much, she throws the "maybe we're better of without eachother" line at me. Over the last few weeks I've been seriously wondering if I should just get out, but then I KNOW she will do something stupid, so it feels like I'm being held as an emotional hostage. Any help or advice would be grately appreciated. Biker1

Thanks for the advice, I'll look in to counseling.