Is my boyfriend telling the truth or am I a mug for giving him another chance?

Hi, me and my boyfriend have been together nearly a year now, the relationships been amazing, we've had our ups and downs but don't we all? Anyway last week we went for a meal, we was going to the pictures afterwards and his friend and his gf was coming. To cut a long story short bascially his friend was bring some weed, i told my bf I didn't want him to have any cause he's had loads lately and I don't like it - he texted his friend saying he doesn't want any so his friend tb sayin u need to grow some , my bf txtd bk sayin 'its either weed or split up' in which his friend replied 'u cheat anyways and how many times you said you can't be arsed any more' ... The can't be arsed bit doesn't bother me as I know we all say it after an argument when were slaggin eachother off to our friends its the cheat part :( his friend said he was jokin afterwards and they both promise its not true.. My bf has swore he hasnt and he wouldn't do that, I've told him to tell me the truth numerous of times and ill even give him another chance if he told me the truth, he still denys it.. I stayed with him but now I can't seem to move on from that? The text keeps repeating its self in my mind! What person sends that text if they don't know they've cheated? You wouldn't, would you? I love my bf more than anything and want it to work, but if its not getting any better and I don't know how to trust him shouldn't I just walk away? Or should I give him the benefit or believing him? The thing is I could be a mug staying with him if he has cheated or if he hasn't and I left him or he's going through all this and he's innocent its awful! The bad thing is, ill never know the truth! Sorry for essay.. Please give some good advice :) xxx