Why would it be any better if he left as opposed opposed cheating?
Realistically you might tell yourself this will hurt you less, but it won't. Because you've kicked yourself while your down if you do it.
Losing someone who you love hurts. But what about self love? Because once that is gone...that's when you are truly lost.
Telling someone you are unattractive and they shouldn't stay with you is far more unnattractive than pretty much anything else that could be going on with you.
Perhaps you should be single. But not for him, for you. Because either you aren't in thr right headspace to be with someone right now OR they aren't the right person to help you feel secure in yourself.
It'd never a partners role to 'fix' you. But some partners can make us feel worse about ourselves (eg: does he bring up his ex a lot?).
You need to decide if you need to be single and do the self work or not. For you. Not for him. Or, if you want to stop beating yourself up and can rely on him to be there for you whilst you gey better.
See your gp about these thoughts. It may be some depression or anxiety medication could help. Or that you would just benefit from having some cognitive therapy.
Id say though...Trust that someone who is with you is with you because they want to be.
As for being more animated, it's normal to be more energetic in short bursts, around ppl you haven't seen in a while. Most people are not full of beans all the time.