I really need help and advice for my relationship

This is going to be very long, I apologize in advance. I'd really appreciate it if you read it all and left advice.

I'm in a very unique situation. I've been with my boyfriend for 3 years, and we've been dating since high school. His junior year I went to college, and after he graduated he went into the Marine Corps. He got stationed on the East Coast, while I go to college on the West Coast. So, we're a military, college, long distance relationship. The odds seem pretty stacked against us, but we're still going strong.

However, this December I recently found out that he had been messaging people on Craigslist personals, sending pictures of himself and even asking to hook up. I know that he didn't meet up though; he was in training at the time, and had no way to actually be able to do these kinds of things. I then found out that he had been on and off dating sites a couple of times throughout 2016, but each time would delete his account. He also only did the Craigslist messaging for a couple of days.

None of these issues have been consistent, they're always very sporadic and don't ever continue on for more than a couple of days. It's not normal behavior. Because we are long distance, we see each other only every few months. I noticed a pattern that he would usually do these kinds of things whenever we had just had a really good vacation and I left. It's around these times that he tells me he feels very depressed. He also told me that when he moved to the East Coast, he was feeling very depressed and homesick. This brings me to another odd stacked against us: His family history of bipolar and mental illness.

I think part of the issue stems from the fact that he might be slightly bipolar. He doesn't experience mania, it's more or less that he feels normal most of the time, then gets into depressive moods every now and then. The military has definitely spurred this, as well as the distance between us.

He has a bad family history, as well. His mom left at a young age and his dad was gone a lot. His parents were never there for him, and he never really had a sense of family. When we first started dating, he ended up having to live with me for 6 months because his dad left. My family gave him a loving environment, and that was when things were happiest between us. We saw each other all the time, we lived together, and things were really good. We had our ups and downs, but we always got through it. He was always very happy.

When I left for college, things started to get rocky. I noticed he wasn't as happy when we would Skype or call, and there were times he sounded depressed. I will also admit that I dealt with the distance very badly, and didn't react well. I would anxiously and profusely text him many times in a row, and would even accuse him of cheating a lot, even though he wasn't. So I know I have faults on my side too. But we've been long distance for a year and a half now, and we've continued to get through this, despite how hard it is. Every time we see each other again, things are amazing, and it's as if none of these bad things ever happened.

I recently went to visit him for Valentine's weekend, and the time we spent together was really incredible. We talked over a lot of our issues, we were very open with each other, we laughed a lot, and things felt very natural and at ease. He told me himself that he felt like we were married that weekend, and that he loved feeling that way. This is how our relationship always feels, despite the times when we're apart. When we're apart, things feel rocky again; it's definitely a lot harder than being in person. Which is true for every long distance relationship.

What I don't understand is that I've found a few more incidents even after I found out what happened in December, and told him to stop. Back in late January, he had texted someone from Craigslist for a few days, but then it ended. Another time was in late February, a few weeks after our trip. With this incident, there were only about 16 texts exchanged, and it only lasted an hour, but it was with someone else from Craigslist, I believe. I also have a genuine instinct that he never went out and did anything. Part of why this messaging ordeal is so hard is because it's completely against his character and personality. Physically cheating seems too against his personality, and I really do believe he hasn't done anything. But of course, having time to sit on it and think really doesn't help. Yet when the thoughts die down, my instinct is still that he didn't physically cheat.

I guess my question is, what do I do? A lot of the people I am close to, who know about the situation, don't ever tell me to breakup with him. They understand we have a very strong relationship and have gone through a lot together in these 3 years of dating and 4 years of friendship. Everyone, myself especially, is just very confused. It's very strange behavior.

When together, things are great. We talk about our future together, and he's never afraid to show how much he loves me. He has always been strongly committed to our future together. His friends know all about me. Even when we're apart, and he's extremely busy because of the military, he always makes a lot of effort to text me. Right now we haven't called in a while, because he's in the field training, but he said once he gets back, we'll get back into Skyping and calling once a day. We also have plans to get married by the end of this year in December; he is about to leave for deployment, and because he doesn't trust his family, he wants to get married that way I'm his beneficiary in case anything happens. I've already been put down as his primary contact for his deployment. This isn't the only reason for getting married, but it is important.

It's just now I'm really confused. I love him, very much. I understand he's had a rough and very lonely life, and now continues to do so with the military and being apart a lot. His behavior of messaging other women isn't consistent, and he has never stayed in continuous contact with any one person. It always ends shortly. If it was more 'normal' so to say, I would definitely draw the line. I respect myself and understand that I shouldn't deal with that.

But to me, this is different. Which is what makes it really hard. I've searched and searched for advice online, but have found nothing similar to my situation. So I decided to reach out by explaining it myself.

What do I do? Please help me. Thanks for reading this whole thing.