Been cheated on twice now and not sure what to do

Hello, I will have been with my girlfriend for 5 years in a few weeks,about 2 years ago we had a bad few months where we fought alot and i cought her kissing a guy who she said was just a friend. I forgave her and we worked past it and things have been going really well since then. Last thursday she went to a friends house and said they were just going to watch some movies for just a girls night, so i didnt think anything of it. It got to be about midnight and i never heard from her all day so i got worried, i never heard from her until 830am the next day when she sent me a text "im home, love you" i was really angry so i called her, when we talked she was acting really quiet and strange, turns out she got super drunk cause her friend had a party and thats why she didnt call, i was really upset because it was supposed to be just her and her friend and they were drinking with a bunch of guys. she never drinks and i didnt understand why she would do this. later that day she told me that when she was drunk she kissed a guy and my heart just sunk, i was so upset and confused. We were talking about getting married and moving in together and i even bought her a ring to suprise her on christmas and then she went and did something like this and just ruined it. she came over today so we could talk and i just felt sick looking at her. When she kissed that guy 2 years ago i got over it easier cause things were bad then in the relationship and it didnt hit me as hard, but this time it hit super hard because of the plans we were making for our life together and things were going perfect and this was outta nowhere.I think to my self if she can cheat on me when things are going well what will stop her from doing it again? both times she said it was only kissing but deep in my soul its hard for me to believe that and it hurts me so bad. She is very closed in and hard to understand what shes thinking sometimes and why she does things, im very open and like to talk so its hard for me to deal with that, leaving me with no clue what to do now. I have a rare disease know as morgellons disease and i am on disability, and sometimes i think i put up with more than i would normally if my life wasnt so hard because she stays with me even know my health isnt good and i dont have alot of money. I try my best to treat her well and show her love but i always end up getting hurt and its hard for me to leave cause i love her very much and and dont want to lose her but i feel so betrayed again. Im just so confused on what to do and if i can ever really trust her anymore. Thanks so much for listening, im just looking for some help to deal with this pain.