Even though they lost a wicket for 6 runs, I don't think it was a big pressure. But they lacked confidence, otherwise they lost the match by 16 runs despite having low target. They had a golden opportunity to win the series but they were not able to do it. Now if something bad happens in the last match, they will have to lose the series at home. I think the Sri Lankan batsmen were a bit nervous, otherwise it was difficult to imagine that they would lose wickets so quickly. Moreover, Sri Lanka is a very strong team at this time. If they can score a little more than 280 or 300 against Bangladesh, then they have a chance of winning. But the way Bangladesh was able to turn in the last match, the team will have a big possibility of winning the next match as well.
What can I say about lack of confidence here, because I don't think the Sri Lankan team lost the match only due to lack of confidence rather I think the bowlers in Bangladesh were able to perform well in this match, which is why such a close result came.
If we look at Shamim Hossain, Mehidy Miraz, Tanvir Islam, all of them have been able to maintain an excellent bowling economy rate due to which the Sri Lankan batters had to come under pressure and ultimately lost the match.