score:11
It will be extremely difficult.
Based on the facts you've explained here, the Entry Clearance Officer processing your application will reason that if he lets you in, there will be essentially no good reason why you would want go back to Belarus instead of staying with your boyfriend in the UK. That is exactly the circumstance where he is supposed to deny a visa.
In all probability, applying for a visa in those circumstances will just be a way to lose the application fee, as well as get a pesky yellow-to-orange flag in your immigration history.
You would need to wait until you're in circumstances where you're more solidly rooted in your home country. Or, you could (plan to) marry and apply for a permit for you to settle with him permanently -- that's not easy to get either, but maybe not quite as hopeless as a visitor visa.