HELLO,everyone!

This is my first post here, as I have come to the point where these issues I am having really impact my life. I am really afraid I might become emotionally unstable amd the fact that I have a family background of mental illness it's not making me feel better..
Sooo, my boyfriend and I have been together for over 2 years and a half. I am 20 now and this has been my first relationship, he is 21 and this has been his first serious relationship as well. He's a great guy, good-looking, funny, caring, smart, loyal, he just felt perfect to me. Everything was amazing, we had plans to spend our life together, a loving, supporting relationship, except that now I have a dilemma that's eating me alive. Although he's been the one to keep my mental sanity over the years, as I have bad family issues, I can't seem to get over something he's done. He chose to go abroad for work for almost 4 months and even if I did not want this and I had some doubts about whether ours will work anymore, I agreed to it in order not to be/look selfish. I really went through a lot of suffering without him, the trauma of seeing a close person practically eaten alive of cancer and me getting unexpectedly really sick, with no clear diagnosis until this day. I missed his love and support and wanted nothing more than to rush the day he gets back. It eventually happened but now three months later I find myself questioning my feelings that I once knew were very strong! It all came on suddenly one day and ever since then I keep having obsessive thoughts that I don't love him anymore and it will all go down the hill. I have ruminated this every living moment for the last 1 month and a half. Sometimes I am inclined to break up with him, as I feel frustrated with being sick and I resent him for his absence when I needed him the most. It is like I found him responsible for my illness and I must accuse him. I keep thinking "maybe we should take a break". I get irritated and it's like I scold him inside my head. I keep looking for the small details in our relationship that did not work out, I keep on checking my feelings when I'm with him to see if I can or cannot feel anything. It's like my mind is trying to confuse me and get me to break up with him! Now everytime I think something negative abouy him/us it just turns into a sign that I don't love him. At the same time, I feel I must not let go of him on a whim. I want to stay with him, I want our relationship to regain its strength, I can't stand the thought of not being with him, of him being with anyone else. He is aware of my struggles and wants everything to work out. But I keep finding myself crying when I tell him these things over and over again. I am just miserable lately. These thoughts are driving me crazy, I feel really depressed and sick, with severe insomnia, but I had low energy, lack of appetite and lack of enjoyment for anything for 4-5 months already, also due to some anxiety issues (I kept thinking I will go sick again, or go crazy and make a scene). I think I might have an anxiety disorder and depression, as I am an emotional rollercoaster, cry a lot, feel unhappy with everything in my life, envy others for being healthy and carefree, have a racing mind, irritability, inability to relax, periods when I feel really uneasy and restless physically and mentally and when I think of just ending everything... I also had some OCD tendencies in my childhood and I don't know if any of these might cause me such emotional confusion all of a sudden. I feel extremely self-centred and I can barely function in my relationships and at keeping up with high standards in my college. It pains me to have such thoughts about my boyfriend, and I know he suffers as well because we used to be perfect for each other and he can't stand to see me so depressed and not communicative, very different from the bubbly girl he knew when he left. If I stay will I forever be filled with these questionable thoughts and resentment towards him? I am really bad at forgiving, break/break up seems unbearable...I really wanna stay by his side, but I feel so confused, and cannot sort out my feelings, get rid of ruminating bothersome breaking up thoughts...
What could this be and what should I do?