I think you need to sort yourself out first, your health. You definitely need to look for counselling, someone who can guide you and help you find the answers. I hope you love your girlfriend and you want the best for her, when 2 people love each other it is wonderful, but don't marry her only because she really wants it or for any other reason, do it when you feel like you want to do it when you know it is time to do it and it is the right thing to do it. Don't decide for her who she should be with, sometimes we don't chose people we fall in love with, respect her feelings and never promise things unless you are sure you can/want make them happen.
There is no need to feel quilt if you are honest to her about everything. I don't think you are in the right state of mind to make big decisions, but be honest about how you feel and definitely look for counselling for yourself. Good luck

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